View Full Version : Preventing Plywood from Checking

Dale Genther
08-25-2010, 11:52 AM
I've always applied a thin layer of fiberglass set in epoxy when making some exterior part out of marine grade fir plywood to prevent checking and it has worked fine. Will a couple coats of just the epoxy without the fiberglass acomplish the same thing? I'm refering to components that do not need the additional strength or abrasion resistance the glass provides.

wizbang 13
08-25-2010, 11:56 AM
I do not think epoxy alone on rotary sawn fir ply will yield the same results as your present superior practice.

Eric D
08-25-2010, 01:05 PM
Not in my experience....

glass is what helps keep it from checking.

08-25-2010, 02:48 PM
imho if its worth building and you want it to last ----why use fir ply to begin with--save it for houses

08-25-2010, 02:58 PM
No direct experience, but someone here was saying that a few coats of CPES would keep fir ply from checking -- hard to believe but possible under certain conditions. But I'd sure be tempted to go for the cloth and epoxy as a sure cure.

Worth a test if you have any scraps of the same ply lying around.

08-25-2010, 03:12 PM
What about a lighter (cheaper) than glass covering. I read that model airplane builders sometimes use cheaper substitutes for Mono-Kote. Don't know about chemical compatibility with epoxy or if the shrink tight finish is lost.

Brian Palmer
08-25-2010, 03:17 PM
My boat is about 20 years old, and was built of fir ply in the decks and cabin. The areas that have epoxy but no glass have checked; those that have glass are fine. Where the glass has been removed because of sanding over the years, I just had to redo the glass to cure the checking (e.g., edge between cabin top and sides). The glass is really important.


Eric D
08-25-2010, 03:41 PM
No direct experience, but someone here was saying that a few coats of CPES would keep fir ply from checking -- hard to believe but possible under certain conditions.

Really? come on.......I am not picking on you Thorne but really??? CPES for checking fir??? I know you love the stuff but this is a bit much and your first comment is you don't know, but then you still advise it.....If full strength epoxy is not up to the job, there is no way diluted, thinned down voodoo juice will be up to the task. A simple search of this forum will show a handful of threads on guys asking the question about fir with only epoxy. Results have always been the same, as demonstrated by Brian above.

Glass and epoxy...good
Epoxy alone.........bad
Paint alone..........bad

sorry, onery day or something but this one just caught me wrong and advising CPES which has limited structural capacity is NEVER going to hold fir from checking.

Take care.

Dale Genther
08-25-2010, 04:59 PM
Ok, that is what I thought. Just hoping to save some time and money, but the risk just isn't worth it.

paul oman
08-25-2010, 05:33 PM
have tried to test this before, but tests didn't survive, got lost etc. Only thing I can add is that there is a huge difference between 1 coat of thinned epoxy and 2 coats. Thinning helps the epoxy soak in a wee bit, which is important. With unthinned epoxy just a surface bond and epoxy skin begins to peel from the edges inward.

08-25-2010, 07:23 PM
Eric - as I said, someone else makes that claim, which I then proceeded to doubt would be effective except under certain conditions. As there are many types of "marine fir ply", ditto for CPES, and double ditto for conditions and overcoatings, I'm willing to believe that it might work in a specialized situation. Sure wouldn't bet a large amount of money against it, as I know very little about the exact structural and material conditions that cause checking.

08-25-2010, 09:32 PM
I've seen two examples where even the glass wasn't enough to hold fir together. On a recommendation in the Searunner manual, I tried Marin-Tex Gluvit on a couple of fir ply hatch covers that were not glassed. It's more flexible than epoxy, so theoretically it doesn't crack open with wood movement. On those two small tests, it held up for the 3 or 4 years after I did that job. But that's not a recommendation to trust it for an entire boat, just a side-note.

David G
08-26-2010, 11:49 PM
Know also that it doesn't take much glass at all to meet the need. I've successfully used as light as 2 ounce glass on fir plywood. No checking at all. The first time was a pickup truck canopy in the 70's. It lived outside 24/7/365 in Eugene, Oregon. Some sun. Lotsa rain. Low's in the 30's. Highs in the 80's. It was painted after glass&epoxy with oil-based porch & deck paint. It lasted for years with no further maintainance and no checking.

On the CPES question - call me an open-minded skeptic. My twin was not making grandiose claims... just noting that someone else had reported success with CPES only. I'd have to see it for myself... or get a report from someone whose veracity and experience were unquestionable. That would allay most of my doubts. Then I'd want to see it for myself. Several successful examples. <G>

Bob Smalser
08-27-2010, 12:29 AM
I'm a big fan of light, 3-4oz fiberglass fabric and epoxy on all new work with plywood not subject to heavy abrasion. I even use it on some solid-wood laminations. Good bang for the buck. It's relatively inexpensive, is forgiving to apply, prevents or retards checking, it stabilizes the surface to prolong the life of the finish, and can even be varnished instead of painted like the 3oz glass on the laminated Hondo tiller yoke below:


I only have one use for CPES, and that's in restorations involving old plywood, which needs all the help it can get. I apply the CPES and instead of waiting a day for it to cure, immediately hotcoat it with a sandable epoxy primer. The CPES soaks in deep and bonds chemically with the thick primer to create what approaches an additional lamination in durability. It won't even chip off in collisions.



Tom Lathrop
08-27-2010, 09:31 AM
In my experience, epoxy alone will not prevent fir ply checking. I've never seen any checking on ply properly coated with glass cloth and epoxy. On the other hand, Dynel will not prevent checking. I've even seen the eye patch surface repairs break through Dynel after several years.

08-27-2010, 10:35 AM
I hope you don't mind if I deviate slightly from the original question. There seems to be a consensus that fir plywood needs fiberglass cloth.

However, do you guys recommend fiberglass on Aquatek or Hydrotek (Meranti hardwood) marine plywood decks? Instead, I'm thinking about just a layer of epoxy, then primer and paint. This plywood supposedly doesn't check as much as fir, right?

I already fiberglassed the bottom of the hull. My question is just regarding the topsides.

What about other exposed plywood, like the cabin sides?

paul oman
08-27-2010, 10:36 AM
wonder how epoxy and cotton fabric - from a bed sheet - (instead of coarse weave fiberglass) would work? it would wet out, have no weave pattern, easy to work with etc. ...

what do you guys think?

wizbang 13
08-27-2010, 11:01 AM
Geary, Wizbang is Aquatek, West and poly varnish. A couple weeks in the sun, lots of small (1/2") checks. Scruff sanding and a few more coats of poly and looks good again. I think it was just the ply stabilizing. The varnished decks are HOT in the sun, but you are painting. I would bet that a light color would work WO glass. But then, if weight and money and labor are not critical, dynel it.

08-27-2010, 11:52 AM
Paul, from my limited experience accidentally soaking my shirt in epoxy, cotton fabric soaked in epoxy is very brittle and weak. Also, cotton can rot. I wouldn't put that on my boat.

Wizbang, thanks for sharing your experience of aquatek w/o fiberglass. Yes, I'm trying to save some money here as I'm slightly over budget.

Eric D
08-27-2010, 01:27 PM
Geary, if you are going to put any epoxy on your boat and worried about budget, truly look at the actual cost of glass. It is SO cheap for light 3-4 oz stuff. just put it on, then put the epoxy over it and think of the savings down the road when you do NOT have to rebuild/repair because of checks.

What is that old bone, do it right once or do it twice!!

08-27-2010, 04:13 PM
I used 2 oz on the exposed surfaces of the boat you see next to my name. no checks in 2 years. 2 oz is great to work with. I used 6 oz on the bottom.

Bob Smalser
08-27-2010, 04:53 PM
This (Meranti hardwood) plywood supposedly doesn't check as much as fir, right?

It's still plywood and needs all the help it can get. Glass it, or you'll dramatically improve its chances of looking like this some day. And your work being cussed by someone trying to restore it.


Todd Bradshaw
08-27-2010, 05:14 PM
When you own a 22' kayak and a 21' garage, something has to give.


This has been up for about nine years now. It's exterior fir ply, coated with two coats of WEST 105/205 epoxy, Scotchbrighted to remove blush but not sanded and then given a couple coats of white Brightside. No glass. It started to check 2-3 years ago. I think the addition of fiberglass does a couple of fairly important things. First of all, just having it there acts as a thickness guage, ensuring that you have adequate resin thickness over the entire surface for sealing purposes. Secondly, the high tensile strength of the glass fibers, low elongation factor and pretty modest crimp in the weave (yarns passing over and under each other) add a fair amount of brute strength to the surface plies to combat expansion and contraction with environmental changes. The less that surface can move, the less likely it is to check and fiberglass reinforced resin is a lot less likely to move and crack than plain resin.

Stretchy fibers (and/or weaves) like Dynel and Xynole would seem less likely to maintain dimensional stability. They wouldn't likely break, but the wood under them probably would, leaving slightly fractured yarns spanning the gaps. Cotton also has a lot of stretch, so it probably wouldn't add much brute force strength and it might not even be strong enough to bridge the gaps over checks unless you went awfully heavy on the fabric weight (and were willing to use a whole lot of resin to saturate it because it's thirsty stuff.

08-27-2010, 06:08 PM
wonder how epoxy and cotton fabric - from a bed sheet - (instead of coarse weave fiberglass) would work? it would wet out, have no weave pattern, easy to work with etc. ...what do you guys think?

Paul - Remember that fiberglass cloth and some other specialized fabrics are treated to take/adhere with epoxy, so there is more than meets the eye here.

The only mention I've heard of using really alternate fabrics is with paint, from the Deep South via Duckworks Yahoo Group, where Chief RedElk (seriously) has described this method for coating cheap ply: Sand ply and coat heavily with latex porch paint, let dry. Put on another thick coat and lay some thin polyester fabric in the wet paint, then paint over that and let dry. Repeat at least once more to get most of the texture out of the surface of the paint. The polyester fabric allows a thicker coating of paint, thus protecting the cheap ply better.

Todd Bradshaw
08-27-2010, 08:19 PM
Cotton is quite commonly used with epoxy (milled fibers) and soaks up resin like crazy with no special treatment. For certain uses, it's great stuff. For use as a laminating fabric though, you can find other fabrics with more desirable strength to weight ratios, more rigidity, more elasticity or just about anything else you might be looking for.

Dave Carnell
08-28-2010, 11:48 AM
Rather surprisingly, a skiff built of B-C fir plywood that I treated with ethylene glycol antifreeze did not check.

Richard Smith
08-28-2010, 08:40 PM
That's what I was thinking as well. I wish time could be accelerated on the EG
treatment theory . . . and my experiments. EG does stabilize wood and I have even mixed it into my own CPES brew. So far, only positive results. The jury is still out on the long term results.