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imported_maguro
05-17-2003, 10:34 PM
I am ready to finish off the inside of my skiff but am confused as to which process to use for the encapsualtion. I already have a gallon of West system epoxy but I'd rather not have to go through all trouble cleaning, sanding and priming with two part (toxic) primers like the interlux 404/414 to help with the compatabiltiy of the single part poly paint that I already have. It sounds like a lot of work to go that route when all is said and done condidering there are lots of nooks and crannies on the inside between frames, stringers and the like. I was thinking of going with a couple of coats of CPES and applying Brightsides directly over it to eliminate the priming and heavy cleaning/sanding process. Is this a better idea or should I stick with the West that I already have?
Thanks

Nicholas Carey
05-18-2003, 03:55 AM
I was thinking of going with a couple of coats of CPES and applying Brightsides directly over it to eliminate the priming and heavy cleaning/sanding process. Is this a better idea or should I stick with the West that I already have?I think you might be confused. Are you talking about painting the boat, or coating it with epoxy?

Even if you coat it with epoxy, you still need to prime it before coating it with gloss.

Primer does a couple of things. One, it acts as a tie coat between the substrate and the finish coat. Two, in modern coating systems (it ain't paint no more), the primer provides the bulk of the coating thickness. The glossy topcoat merely finishes things up.

Get a copy of Interlux's Painting Guide—http://www.interlux.com/—you'll find it useful.

Scott Rosen
05-18-2003, 06:33 AM
Could you tell us a little more about the construction of your boat? Is it carvel, strip molded, lap, or something else?

imported_maguro
05-18-2003, 09:29 AM
Nicholas, Yes I am confused. I always thought CPES acted as kind of a primer itself. At least if I do use CPES I hope can get away with a one part primer and it will cut down on the work time without sacrificing any durability or watertightness. Am I correct in assuming this?

Scott, my boat is sheet planked with 3/8" marine doug fir (already checking after one year of construction). Franes are honduran mahogany, stringers and chines are Meranti and keelson and knees are white oak. Joints are at the chines and keel.

MarkC
05-18-2003, 11:49 AM
Do a search for 'paint and epoxy'approx. 3 month's ago a discussion - I believe I read that some people called the info line on their paint tin (Kirby's I think - a US brand) and they advised their top coat was compatable with epoxy and that no primer or undercoat was necessary over the epoxy. But then, for filling scratches in the sanded epoxy??

mark

Wild Wassa
05-18-2003, 02:18 PM
Originally posted by maguro:
"I always thought CPES acted as kind of a primer itself. At least if I do use CPES I hope can get away with a one part primer and it will cut down on the work time without sacrificing any durability or watertightness. Am I correct in assuming this?

Maguro, Here are a few details that I have pulled from Smith and Co's notes on using CPES.

CPES is an excellent adhesion-promoting primer for paint when the top coat needs a primer.

Before painting over fillers, apply CPES to help the paint bond better to the wood and filler.

Below the waterline, over CPES apply epoxy as a barrier coating.

Above the waterline, Apply CPES to the entire surface. If finish is bright apply varnish or polyurethane.

Do not apply fillers over freshly impregnated wood, the solvents in CPES must be allowed to evaporate.

My note: I allowed CPES to cure for 8 days prior to coating. It was a long wait, but with epoxy curing I'm use to waiting, I use to use TPRDA, ... "thirty days your Honour?" "Take him down." CPES is such an easy sealer to apply it is almost criminal.

Warren.

ps, I can see why people like CPES, having now used it on a rotten old plywood hull. Where did the rot go ?

[ 05-18-2003, 02:57 PM: Message edited by: Wild Wassa ]

Scott Rosen
05-18-2003, 08:29 PM
Warren,

Don't you find that CPES works exceptionally well on old, dried out and maybe even rotten wood?

I've used CPES and Fill-It for a number of repairs, and I've been extremely pleased. It's true that the CPES restores the strength to the wood.

For painting old wood, I've found that CPES followed by a two-part LPU primer gives outstanding results. Use the topcoat of your choice.

I'm doing my cabin overhead soon, which is mohogany ply. I plan to use CPES, followed by Interlux 404/414 (epoxy easy-to-sand primer) followed by two-part LPU topcoat. It's an unpleasant job, but I'm hoping I won't have to do again for at least 10 years.

Wild Wassa
05-19-2003, 12:48 AM
Scott, I had a small play with CPES on new marine ply earlier this year, this is different on real rot. Iím really pleased so far. Two ply panels above the chine were soft. I left them on the boat. I did this to see how it would respond to the sealer, if it wasnít going to work now, they were off the boat. Iím so pleased that I left them on. The original quality of the boat is extremely high, the joinery is of fine furniture level with marine 5 ply, Iím so pleased that I didnít remove the panels, I would have struggled with it to keep the quality. I only left the panels on to show Tony H how bad the rot was. The first thing he said was, "this is what CPES is designed for."

The resin matrix that is formed by CPES's penetration so far is excellent. It has gone from softish to rock solid in some spots, some bits became brittle and I reworked them. The epoxy responded well, it is hugging the wood, this is new to me too, and I trowel it on, :rolleyes: . The ease if application was another big advantage after normally using epoxy with TPRDA. I had no control over the soak-in of the CPES. I put a lot on. The areas that didnít form a light gloss got soaked. I used 1.7lts. I put 5 coats on the areas of rot, it didnít stop soaking in, even the epoxy thinks it can soak in, now. With Epoxy and TPRDA, I would have used about 700mls, and would have needed to remove the two panels; they were too soft or cut-in too far. Now that I have used CPES I realize that I may have removed more wood than I needed. I'll know next time.

The epoxy coat is now on and I'm fairing with glass macros. Larger holes in the hull I'm still doing (or have done) with marine ply, tape, epoxy and Fill-it. Fairing the repairs should be in only a few days from now with the Fill-it. I do like the way Fill-it peaks.

I also like the curing of the glues, fillers and paints closer to their polymerization, before cutting, even though Fill-t is sandable after 24hrs, as you are aware. Just something that I learnt in the Dark Ages, it seems to suit the way I work also. Thatís my excuse for being a slow worker.

Everything that I have read about CPES's fine properties I concur with, so far. I look forward to watching CPES age. That's the test now. The deck is worse than the hull, this is still to be CPESed. If this boat holds together with this much plastic in her, ... it is going to be interesting to see. Wild Heidi her name is.

Warren.

ps, I wish epoxy cured more like a 30 minute Araldite, that would be good.

[ 05-19-2003, 01:56 AM: Message edited by: Wild Wassa ]

Scott Rosen
05-19-2003, 06:27 AM
Warren,

As you noticed, the Fill-It is easy to sand for the first 24 to 48 hours. After that, it gets more difficult.

I've had a few situations where I needed a slightly more viscous filler. Smith told me to "thin" the Fill-It with a small amount of his Layup and Laminating Epoxy. That, too, worked great.

Wild Wassa
05-19-2003, 06:53 AM
Scott, I don't use sander fillers with epoxy, I find them brittle and soft. So I have resorted to using the compression strenghth fillers as my norm. Sanding is no problem, hard fillers suit. Thanks for the warning, I felt the heart flutter.

I fashion my own epoxy laminate. I understand the principle of Chobbam Armour (the state of the art) as used on Abram's Main Battle Tanks, and having learnt to paint structually, with oils over the other media, 'I'd like to think' (but time will tell) that I could be there already. The dinghies are only 3 or 5 ply, so far. Chobbam armour on a dinghy. :rolleyes: .

Epoxy is somewhat brittle by it self (Scott, I only post for the novices like myself). Fashioning the laminates saves me much hard work and makes the pieces that I insert, with the compression fillers in the glues, very strong. The reason I like the little dinghy racers is they are fragile, and very managable, so the laminates are proportional to the wood that has been replaced. Fairing is the real fun, and very enjoyable.

Warren.

ps, I'll fill an area tomorrow with more Fill-it, then I'll sand after 24 hours. That will give me a chance to compare the difference against the long cure, thanks for the extra info.

[ 05-19-2003, 07:22 AM: Message edited by: Wild Wassa ]

Nicholas Carey
05-19-2003, 02:39 PM
Originally posted by maguro:
Nicholas, Yes I am confused. I always thought CPES acted as kind of a primer itself. At least if I do use CPES I hope can get away with a one part primer and it will cut down on the work time without sacrificing any durability or watertightness. Am I correct in assuming this?Well...I've never had any problems with applying a one-part primer over epoxy. I'm not sure why you feel you need a two part primer, if you're putting down one-part paint. Just make sure you scrub the cured epoxy with a scotch-brite pad and water first to remove any amine blush. You might want to sand the epoxy with 80 grit to give the primer something to grab onto.

As far as cutting down work time, goes, I don't think there's any good way to significantly reduce the time it takes to build a quality, good-looking finish.

With modern paints, as I pointed out earlier, the bulk of the overall coating's thickness is provided by the primer and not by the topcoat. Hence, the primer is critical to a good job. On the plus side, the primer should sand easier than the topcoat. It's soft and intended to be sanded.

To get the good-looking paint job you want, you build up with primer—a minimum of two coats—sanding between coats until you have a nice fair, smooth and uniformly colored surface. Any nicks or dings in the surface will be very visible once the glossy topcoat is applied, as will any unfairness in the surface. Any 'holes' or blotches in the primer coat are likely to telegraph through the topcoat and show up as darker or lighter areas. This is especially true of reds and yellows and not so much with whites or blues. It's a function of the pigments used to color the paint. All this prep work is key to making the final coat look good. A good friend of mine does french polishing (padding on hundreds of microscopically thin coats of shellac for a bright finish). He tells me (and I know it from varnishing) that the hardest things in the world to finish well are broad flat surfaces like a table top; the easiest are things like a carved leg with a lot of relief. On a broad flat surface, any imperfection sticks out like a wart; on a carved table leg or a narrow, tightly curved surface, imperfections in the finish tend to be nearly invisible.

You can use a 'high-build' primer like Z*Spar's 105 Undercoater or Interlux's Brightsides to build thickness faster. Before you start painting and when you sand petween coats, make sure you fill in any little dings or nicks with a glazing putty—either epoxy with microballoons or a product like Interlux's Surfacing Putty.

On the second coat of primer, you might want to add a little color to it by adding a little bit of your topcoat paint or japan colors. The addition of color to the second coat helps you in your sanding—it's easy to see where your high spots are.

Once you get the perfectly smooth, well-primed surface, it's time for the topcoat. You'll want at least two coats.

There are, I believe, no shortcuts in achieving a quality paint job. A quality paint job depends on quality surface prep. Quality surface prep takes time.

thechemist
05-19-2003, 03:13 PM
The word "Primer" has many meanings, according to the use or function of the primer.

The build-up of a coating film thickness is one use. It is distinctly a different use than impregnation of a porous surface, to prevent further coatings from soaking in and telegraphing the grain to a topcoat.

A further different kind of primer is one that promotes adhesion.

Oil-base primers, for instance, have no chemical adhesion mechanism to bond them to fully-cured epoxy-amine systems. CPES is evidently not that kind of epoxy, for it is reported to improve adhesion of such primers or topcoats.

There are many different adhesion-mechanisms, one of the oldest being to a rough-sanded surface, by "tooth". Chemical bonding is another adhesion-mechanism. There are many different kinds of chemical adhesion mechanisms that a formulator may employ in the design of coating systems. One of those is to have the same reactive chemical groups in both coatings.

Latex primers or topcoats may have double-bonds built into their structure, as do oil-base primers , oil-base topcoats or varnishes. Since both of those cure by the oxidation of double-bonds to ethers, common chemical linkages can be established between that kind of primer and topcoat.

A topcoat is normally selected for its visual properties and sun/weather resistance. Where the desired topcoat does not have the desired mechanical properties, a "primer" is often used below the topcoat to provide the some of the mechanical properties desired of the coating system as a whole.

Bruce Hooke
05-19-2003, 04:01 PM
While others have addressed the primer issue I think there is another issue that you should be focusing on first. CPES and West really have somewhat different functions:

CPES is, by all reports, a great way to prepare raw wood for painting. It seems to reduce rot and improve the adhesion of paint applied on top of it. However, based on everything I've seen it will not do that much to prevent water from being absorbed into the wood. Also, it certainly will not stop douglas fir plywood from checking and it will not cover existing checks. CPES is easy to apply, but you do need a space with good ventilation.

Epoxy coating, on the other hand, does some things besides help with paint adhesion. In my thinking, the primary reason for epoxy coating is to encapsulate the wood and make it very hard for rot to get started in the wood by preventing water and oxygen from getting into the wood. This, of course, only applies if all sides of a piece of wood are coated. Also, epoxy is harder than most woods, and more abrasion resistant, so it will make for a more durable surface. Epoxy will stop checking in douglas fir plywood but only if you use medium-weight fiberglass cloth along with the epoxy. Some paints have trouble sticking to epoxy, and others do not. Some testing is in order here (or try to get input from the paint manufacturer). Done correctly, paint over epoxy can last much better than paint over raw wood (I'd hesitate to say how it compares with paint over CPES). The best paint to use over epoxy for long-term durability appears to be two-part (LP) paints. (As an aside, these paints are very glossy, which some wooden boat owners don't like.) These paints also require more skill to apply well, and you need to have very good ventilation when you are applying them. Based on what I've seen single part paints do also last longer over epoxy than they will over raw wood. This is, I suspect, in large part because the epoxy pretty nearly stops any water from getting into the wood, which is a major reason for pealing paint.

Based on the above, epoxy may sound like a great way to go. However, in your situation there is a major down side. Gougeon Brothers (WEST System) recommmend applying three coats of epoxy in most situations and sanding between each coat, or at least after the second and third coats. Applying epoxy and sanding it on an already completed hull interior is a major pain-in-the-you-know-what. Much the best way to encasulate with epoxy is to do it as the boat is being built so that as much as possible parts are coated and sanded outside the boat. Coating an already completed boat interior with epoxy is such a pain that I would not recommend it unless there is a very compeling reason for doing so, such as that you want to try to get a show-quality finish on douglas fir plywood. (Of course the better way to get to that end would have been not to use douglas fir plywood in the first place.) So, in your situation I would almost certainly go with CPES...

thechemist
05-19-2003, 06:32 PM
Originally posted by Bruce Hooke:
<snip> In my thinking, the primary reason for epoxy coating is to encapsulate the wood and make it very hard for rot to get started in the wood by preventing water and oxygen from getting into the wood. This, of course, only applies if all sides of a piece of wood are coated. <snip>This idea has been promoted by the Gougeons originally and for over thirty years. In some circumstances it can be a trap.

Oxygen WILL diffuse through epoxy coatings, and fairly easily at that. It is a small molecule. So is the water molecule. It, too, will diffuse through epoxy coatings. Where those epoxy coatings are rendered even more porous with age than initially, by the loss of volatile plasticizers, water and oxygen diffusion will be higher. So, those coatings are only RELATIVE barriers, not absolute ones. They slow the rate of diffusion, but don't stop it.

Here's the problem: In the real world, those epoxy coatings, without reinforcing fibers such as a layer or two of glass cloth, crack. Eventually, inevitably, they crack. The stiffer ones, such as West, or those which contain volatile plasticizer/diluents that eventually diffuse out of the coating film, leaving it even more brittle [Such as West] and more likely to crack under stress.

The crack is bad because liquid water can enter. Eighteen grams of water occupies eighteen cubic centimeters as liquid, but 22,400 cubic centimeters as vapor. Thus , water gets in easily but gets out slowly.

When water gets in through a crack, it stays in a long time....long enough for rot to start in the vicinity of the crack.

The solution to this is to make it difficult for water to get in from the outside, but to have a relatively light coating on the inside of the boat so water in the wood can easily evaporate off the interior surfaces.

In other words, Don't encapsulate the wood. Just put barrier-coatings where they are actually needed.

This came up before....Here:
http://media5.hypernet.com/cgi-bin/UBB/ultimatebb.cgi?ubb=get_topic&f=1&t=003575&p=

Bruce Hooke
05-20-2003, 10:47 AM
Chemist -- I've certainly heard that argument before and read and added to many threads in the past on the matter. You do but some better details in to flesh it out. I guess what leaves me suspicious is that there are a lot of epoxy encasulated wood boats out there now and I have not heard about major problems with them rotting from the inside out. If the process you describe is happening on a widespread basis then it seems like many of the cold molded boats out there should be crumbling into piles of rotten wood. Certainly epoxy will allow oxygen in; I wonder if it allows enough in to sustain serious rot? Or maybe the wood gets too saturated with water and stays that way, which also prevents rot. In any case, whatever the process, there seems to be ample proof that epoxy encapsulation works.

Also, since fresh water is worse than salt water as far as rot goes, it would seem like a poor idea to leave the bilge exposed to fresh water but do one's best to keep out salt water on the outside.

Scott Rosen
05-20-2003, 11:18 AM
Bruce,

I think there's a world of difference between a cold-molded boat, on the one hand, and a traditionally built boat with epoxy encapsulated pieces, on the other hand.

If there's a crack in the outer coating of a cold-molded hull, any water that gets in will likely be localized, and will not attack a structural member, as the hull is essentially one piece.

Bruce Hooke
05-21-2003, 10:43 AM
Scott,

You do have a point there that is worth keeping in mind, but I would argue that:

1. Any plywood is pretty close to pre-made cold molding - the glue lines act as water barriers and water is relatively slow to travel far across the grain.

2. Even cold-molded boats have many structural members that are up to 3/4" thick (per laminate) and relatively wide. Thus, the in terms of the sizes of a piece of wood involved, a cold-molded boat is not that different from, say, a "traditional" hard-chine plywood boat; and I just have not heard about lots of problems with either rotting away in a hurry.

I certainly would agree that any method of construction that depends on wood swelling for the boat to be watertight is a very poor choice for encapsulation. Plank-on-frame and traditional lapstrake should not be encapsulated, but I would say that this is not because of rot but because in these methods the wood must be allowed to take on water...

thechemist
05-21-2003, 12:13 PM
Originally posted by Bruce Hooke:
<snip>
1. Any plywood is pretty close to pre-made cold molding - the glue lines act as water barriers and water is relatively slow to travel far across the grain.
<snip>I actually sectioned a variety of plywoods and examined them under a 25X microscope, and you might do it, too.

That is not true. The glue lines are actually quite variable in thickness, as one winter growth ring bites into a softer summer growth ring randomly adjacent. Anyone can saw-cut, sand, look and see this.

Glue line thickness ranges from about five mils to zero and there are often complete gaps. Spruce plywood with its more uniform grain shows this less, and fir plywood much more.

Steve Paskey
05-21-2003, 12:33 PM
Regarding the potentially bad effects of epoxy encapsulation . . . I used to own an 18 foot sailing dory built by Lowell boat shop. The bottom and side planking were pine, except for a ply garboard. The bottom and garboard were sheathed in epoxy inside and out, including a relatively thick layer on the inside bottom. The inside bottom developed a bunch of cracks, standing water got in, and it didn't take long before the areas around the cracks had turned to mush. Rot was also beginning to develop along the seam between the garboards and the epoxy-less planks directly above them.

I bought the boat from the Alexandria Seaport Foundation, which sold it as having been "restored." The cracks were there when I bought it, under a fresh coat of paint. Needless to say, they took it back for a full refund.

[ 05-21-2003, 12:35 PM: Message edited by: Steve Paskey ]

Wild Wassa
05-22-2003, 11:23 PM
http://www.imagestation.com/picture/sraid63/p87e74b2e57b0b4d737ca575e5555bd4a/fc1873db.jpg

Sea Scouts know about trapping water, even the sander filler has gone mouldy. Rippled decks are my favourites.

Warren.

ps, On Ish's thread about superstitions, ... blue is bad luck on a boat.

[ 05-22-2003, 11:28 PM: Message edited by: Wild Wassa ]

Bruce Hooke
05-23-2003, 01:17 AM
Originally posted by thechemist:
</font><blockquote>quote:</font><hr />Originally posted by Bruce Hooke:
&lt;snip&gt;
1. Any plywood is pretty close to pre-made cold molding - the glue lines act as water barriers and water is relatively slow to travel far across the grain.
&lt;snip&gt;I actually sectioned a variety of plywoods and examined them under a 25X microscope, and you might do it, too.

That is not true. The glue lines are actually quite variable in thickness, as one winter growth ring bites into a softer summer growth ring randomly adjacent. Anyone can saw-cut, sand, look and see this.

Glue line thickness ranges from about five mils to zero and there are often complete gaps. Spruce plywood with its more uniform grain shows this less, and fir plywood much more.</font>[/QUOTE]This is interesting, but I'm not sure how far it goes to answering the question of how effective the glue joint in plywood is at stopping water migration, compared to how effective an epoxy joint in cold molding is at doing the same thing.

Bruce Hooke
05-23-2003, 01:20 AM
Originally posted by Steve Paskey:
Regarding the potentially bad effects of epoxy encapsulation . . . I used to own an 18 foot sailing dory built by Lowell boat shop. The bottom and side planking were pine, except for a ply garboard. The bottom and garboard were sheathed in epoxy inside and out, including a relatively thick layer on the inside bottom. The inside bottom developed a bunch of cracks, standing water got in, and it didn't take long before the areas around the cracks had turned to mush. Rot was also beginning to develop along the seam between the garboards and the epoxy-less planks directly above them.

I bought the boat from the Alexandria Seaport Foundation, which sold it as having been "restored." The cracks were there when I bought it, under a fresh coat of paint. Needless to say, they took it back for a full refund.Sounds to me like a good example of why boats built using traditional methods are not such good candidates for encapsulation. It sounds like the planks may have been moving around more than the epoxy could handle without developing substantial cracks...

imported_maguro
05-23-2003, 07:39 AM
Sounds like there will always be issues with encapsulating but peace of mind goes a long way as well. I'm kind of thinking of it as the placebo effect...if I trully beleive encapsulation is going to work...maybe it will smile.gif
Thanks for the informative posts!

thechemist
05-23-2003, 11:44 AM
Originally posted by Bruce Hooke:
</font><blockquote>quote:</font><hr />Originally posted by thechemist:
</font><blockquote>quote:</font><hr />Originally posted by Bruce Hooke:
&lt;snip&gt;
1. Any plywood is pretty close to pre-made cold molding - the glue lines act as water barriers and water is relatively slow to travel far across the grain.
&lt;snip&gt;I actually sectioned a variety of plywoods and examined them under a 25X microscope, and you might do it, too.

That is not true. The glue lines are actually quite variable in thickness, as one winter growth ring bites into a softer summer growth ring randomly adjacent. Anyone can saw-cut, sand, look and see this.

Glue line thickness ranges from about five mils to zero and there are often complete gaps. Spruce plywood with its more uniform grain shows this less, and fir plywood much more.</font>[/QUOTE]This is interesting, but I'm not sure how far it goes to answering the question of how effective the glue joint in plywood is at stopping water migration, compared to how effective an epoxy joint in cold molding is at doing the same thing.</font>[/QUOTE]I thought it did answer the question. Let me say it another way. The glue lines in plywood are not intact five-mil moisture-diffusion-resistant barriers. They do not exist as continuous, intact films. You can assume water will migrate through plywood, although not as fast as a solid piece of lumber, because the glue lines have erratic thicknesses, by actual observation. In places they have effectively zero film thickness and are, for all practical purposes, wood-touching-wood. This is not an urban legend. Anyone with a microscope, a belt sander and a flat surface upon which to put a piece of 120 and 220 sandpaper can make a clean-sanded plywood section and observe this for themselves.

Thus, plywood gets a score of close to zero in my opinion. Epoxy.....well, that's another story.

An epoxy-joint may be thicker, but it *Does* depend on you and your epoxy. Color some epoxy and make some joints of your own and section them similarly and see for yourself how yours do.

If you want a controlled-film-thickness joint, you'd have to put a layer of glass cloth in there. Even then, the glass fibers would mash down in spots.

Bruce Hooke
05-23-2003, 12:42 PM
Chemist - In a sense you have made my exact point. As I read what you are saying, neither epoxy nor the glue joints in plywood can be considered an absolute water barrier. So, it seems to me that the two can be considered roughly analogous and since there seems to be ample evidence that cold molding works (i.e., despite inevitable breaches of the outer epoxy coating, cold molded boats do not seem to rot away in a hurry), it seems clear that encapsulated plywood should also hold up well. (The key word in my previous post was "compared").

John Blazy
05-23-2003, 02:28 PM
Nice educational exchange guys - thanks. All encapsulating (plywood) is really as strong as the weakest link, or thinnest, most-vulnerable-to-piercing area. Which means that the best method applied in a poor to fair way can be worse than the worst method.

Seems to me that people here should be preaching thick, puncture-proof fiberglassed whole bottoms or at least the joints and outside corners (30-60 mils using around 40 ounces of cloth/CSM) more so than the finer differences between epoxies and primers.

Of course, I understand that it is assumed that the reader will use common sense application/construction methods, but I know that scraping bottom over a rock just once will play havoc on the best epoxy/glass/primer/2K urethane job!!! :D
Or am I the only one that intends to take his handmade boat in shallow, rock infested waters?

paul oman
05-24-2003, 10:30 PM
I sell all kinds of epoxies that I have specially formulated, but I don't pretend to have all the answers, in fact, I guess this post is something of a question (or comment with a request for responses)........

pls comment on the following 'thought'.....

Wood (at least some woods) expands and contracts more than epoxies. By epoxy coating just one side of the wood you remove its ability to expand or contract on that surface, but not on the other un-epoxies surface. This can do strange warping things to the wood..... (right or wrong??)

paul oman
www.epoxyproducts.com/marine.html (http://www.epoxyproducts.com/marine.html)

John Blazy
05-24-2003, 11:58 PM
Paul,
Your concept is correct, but there are quite a few other forces at work, and are significant at certain threshold limits, and the overlap of these limits. First, the rules of the movement game:
- Solid wood expands and contracts based on moister level absorption (no other factor), and is strong enough of a force to sheer delam epoxy/glass sheathing only at the threshold point where the thickness of the wood creates a collective strength to overcome the bond of the epoxy to the wood.
- Plywood does not expand or contract - not significantly enough to be of any note in the movement game
- Epoxy expands and contracts like a thermoplastic based on heat load cycling - I've tested this when I was a chemist.
- fiberglass cloth/mat is about as dimensionally stable as the plywood, which holds the epoxy down pretty good from thermal movement as well.

So based on these rules, as an answer to your question, the primary influence epoxy has on thick wood (like planking), is not the strength of it "locking" the surface into dimensional stability, but in sealing it from moisture absorption. Thats why you wouldn't want to glass over thick planking, unless you know that the wood will never see moisture, hasn't had any in it, and the other side is equally glassed.

Thin wood, like in cold moulding or plywood, the epoxy/glass can be strong enough to "lock" the surface into dimensional stability.

Its not a question of "Wood (at least some woods) expands and contracts more than epoxies", but rather wood expands/contracts differently, by different forces than epoxies.

The other wrench of movement thrown into the mix, is the movement of post cure shrinkage of the epoxy (some more than others) due to further crosslinking of the molecules and/or the "drying" out of the polymer by the off-gassing of unreacted monomers or liquid constituents that never bonded in the molecular matrix, and will migrate out over time, causing brittleness and shrinking - creating sheer forces. This is much more apparent in other polymers rather than epoxies, like polyesters and urethane topcoats.

thechemist
05-25-2003, 12:29 PM
Originally posted by John Blazy:
Paul,
&lt;snip&gt;
The other wrench of movement thrown into the mix, is the movement of post cure shrinkage of the epoxy (some more than others) due to further crosslinking of the molecules and/or the "drying" out of the polymer by the off-gassing of unreacted monomers or liquid constituents that never bonded in the molecular matrix, and will migrate out over time, causing brittleness and shrinking - creating sheer [edited: shear] forces. This is much more apparent in other polymers rather than epoxies, like polyesters and urethane topcoats.Good comment, John. However, the post-cure shrinkage can be apparent in epoxies as well, especially those that use volatile plasticizers. The most volatile of these I know of, that is in common use, is Benzyl Alcohol.........so volatile that the California Air Resources Board considers it a VOC [Volatile Organic Compound]. West has it, as well as most cycloaliphatic curing agents.

The Wood Handbook http://www.fpl.fs.fed.us/documnts/FPLGTR/fplgtr113/fplgtr113.htm is one of the best and most readable wood references I know of.