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JimD
07-06-2012, 08:39 AM
A new forumite messaged me and asked me about these products. I've never used either but I did find this from a Duckworks article and wondered if anyone had any personal experiences to share. He has in mind the Lumber Yard Skiff.

MDO and HDO are special use panels that were created by the plywood manufacturers to do some specific jobs, but have also found acceptance and interest among boatbuilders and in other crafts. The initials MDO and HDO refer to either Medium Density Overlay or High Density Overlay. The overlay they’re talking about comes in the form of a phenolic (heat activated) resin impregnated paper that is laminated to the faces of a plywood panel during the manufacturing process. Medium and High simply refer to the weight and thickness of the paper panel face, but there are notable differences between these two as well.
Originally MDO was created to satisfy the requirements of long lasting Interstate Highway signs. It’s usually produced to rigid specifications from very high grade materials and is supposed to last almost indefinitely in extreme weather conditions with no checking and with only a coating of paint (usually two-part epoxy) for protection. The panel is composed of fir cores and centers with the overlay over well-sanded knot-free faces. Nowadays, the faces are only fir, but in the past thin veneers of luan were used. The reason for the luan faces was to create an even smoother surface, which prevents the grain of the fir from telegraphing its pattern through to the surface. The same process is used for laminating fine-expensive wood veneer faces like teak over fir plywood, but of course without the phenolic overlay.
The surfaces of the MDO are waterproof, but stippled to readily accept and retain paint. The overlay is impossible to separate from the plywood, and if the edges are properly sealed the panel will last practically forever.
MDO is a good product to use for small boat hulls because if it’s used properly, the edges sealed and seams taped, sheathing large areas of the exterior may not be needed.
HDO on the other hand up until recently, wasn’t a good material to use for boatbuilding. While HDO is even more indestructible than MDO, paint won’t stick to it and not much else will either. The panel construction process is the same, the paper is a heavy phenolic-coated paper but the product is generally used to build concrete forms and molds. The surface is made slick (I believe with a castor-oil based mold release) so that the forms and panels can be used over and over again.
Now, however, Simpson Timber has come up with an HDO product using a heavier overlay, but designed to accept paint and reflective tape and is to be used in similar applications as MDO. It’s called “Highway HDO” and in addition, its heavier overlay contains a greater percentage of resin for increased moisture resistance.
Whether MDO or the new HDO is for your small boat project or not is something that I can’t know or recommend. It’s something you’ll have to decide, but it’s good to know that products like this exist for those times you may have an application where MDO or the new HDO offering would be just right.
If you work and build with plywood, product knowledge is not only important, but also comforting to have, and for this reason I can’t recommend the following websites highly enough. There are great product lists, specifications and product informational files that can be downloaded in both word.doc and acrobat.pdf formats. They’re great resources and should answer any questions you may have.
American Plywood Association (APA) Home Page:
http://www.apawood.org/
Simpson Timber Products Homepage:
http://www.simpson-plywood.com/products.htm
Simpson Plywood Homepage:
http://www.simpson-plywood.com/


From http://www.duckworksmagazine.com/01/columns/slogging/09/mdohdo.htm

John Bell
07-06-2012, 08:43 AM
I built one boat out of MDO and would do it again. It was easy to work with and finish. Last I heard was that 10 years later the boat was still going strong. Biggest problem with MDO is the thinnest stuff is 3/8" (9mm), so it's only suitable for larger boats.

David G
07-06-2012, 09:21 AM
I've used a lot of MDO over the years - with good results. Just finished restoring a 50's vintage plywood runabout where the aft 4' of the bottom was replaced with 3/8" MDO. I used G1S, turning the papered face inside, and glassing the exterior (fir) face. My main supplier is Olympic Panel, and they have it down to 5/16" thickness... but no 1/4". Wasn't aware of the new Simpson paintable HDO. Beware, though. There is some imported "MDO" that uses the same sort of phenolic/paper facing on panels of very non-durable exotic species. It's lighter in weight... which is nice. But I wouldn't trust it to be long-lasting.

John Bell
07-06-2012, 09:58 AM
IRRC, Peter Lenihan (of "Just fore the fun of it" thread fame) built his boat out of Simpson MDO.

potomac
07-06-2012, 12:06 PM
I believe Spokaloo built his Lumber Yard Skiff from MDO and has been very pleased. I am currently finishing up a SOF SUP wherein I used MDO for the frames. I believe Duraply is the brand. Working it has been uneventful. However, I do have to report that I found multiple voids.

Christopher Morgan
07-06-2012, 01:47 PM
I too found some voids -- but I think it may depend on the supplier and/or the sheet. The sheets I bought for my first project with it seemed to have fewer voids than the sheets I got the second time.

gstanfield
07-06-2012, 02:24 PM
I've used MDO on a few projects and I think it's pretty good stuff, especially if you are like me and live somewhere that good materials is just not avaliable without paying insane freight charges on plywood panels. As mentioned above MDO will have a few voids, but it's still plenty strong and the ability to paint directly to it is wonderful!

Spokaloo
07-06-2012, 02:48 PM
I have done as much as physically possible to maim, destroy, and otherwise damage my LYS made out of MDO. I used it on everything, even getting 1 sided MDO for the transom and laminating the wood sides together. MDO lasts well, takes abuse well, and depending on the source, can be had with relatively decent cores. Another huge benefit in a fast build boat like the LYS is that it takes NOTHING to finish it. Literally one coat of primer to fill the paper a bit and it was ready for paint. I couldn't recommend it enough for the lumberyard skiff.

Clippercraft boats of Portland made their bottom planking and side decks of MDO for decades, and I can think of several early '70s boats still out in the albacore fishery. Chris Craft did it for years too in the Sea Skiff models.

E

seo
07-06-2012, 03:33 PM
I used to a lot of MDO on the superstructure of a 64' passenger boat I owned. In my area there were three lumberyards that I did business with, and I learned that their MDO was not made by the same manufacturer, and was of very different quality. They were also slightly different thickness, by 1/32" or so, not much, but enough to make a butt joint pretty conspicuous.
My suggestion would be to visit the lumberyards in your area, and look at their pile of MDO. Count the number of plys in a sheet, and check for voids. All other things being equal, more plys are better. In any case, voids are bad.

Todd D
07-06-2012, 07:01 PM
I used MDO for my interior. So far it is great. It does really like paint though. The first coat essentially disappeared into the surface. It took a minimum of three coats to get a decent finish. I have had a scrap of G2S 3/4" MDO sitting in my yard next to the wood pile for the last two years. It still looks like new.

csroy8014
07-06-2012, 07:42 PM
JimD:

thanks for starting this post. I am looking to build a LYS soon and MDO seemed like it would be quite a savings over marine plywood. I would probably put some coats of epoxy on the entire hull (inside and out). Sounds like it would be a good option. I saw a great 18' LYS on another post (extrapolated from 16 and 20' plans). Anyone have any idea how that adaptation was made?

Thanks again.

Bluegill
07-06-2012, 08:09 PM
How much does MDO cost ?? How much does marine grade douglas fir cost ??

potomac
07-06-2012, 08:56 PM
1/2'' MDO= $39/ sheet. 1/2'' Fir Ply= $70/ sheet. Here in St. Augustine.

Fir is tough to paint. MDO is made to be painted.

pipefitter
07-06-2012, 09:03 PM
How much does MDO cost ?? How much does marine grade douglas fir cost ??

Marine grade D-fir ply, unless the designation has changed, was classified as "Marine Exterior". Which means it was for the exterior of buildings at the waterfront and not specifically boat building. It was just a more weather proof panel than typical construction plywood, and when used in conjunction with some of the more hardy marine coatings such as polyester resin with fiberglass, was just good enough for boat building. The elder marine exterior fir also had more plies of equal thickness. That of which I have seen lately, has a thin outer veneer like many marine rated panels, which does alter the structural rigidity of the panel with reference to grain orientation.

Last I checked, the prices were comparable. If enough people use it for boats and the M in MDO gets changed to Marine Density overlay, I would expect the price to double.

Even if MDO was quite a bit more expensive, the amount of finishing it takes to get reg fir ply to a decent finish, just considering the checking alone, the economy of using MDO becomes apparent nearly instantly.

MDO is pretty heavy, but is a great choice for designs converted from more traditional building practices, where the extra weight of heavier traditional materials affected the performance of the hull. It's a trend these days to build light, but is not always best when trying to include certain performance attributes of the older designs.

Spokaloo
07-07-2012, 11:24 AM
Don't waste epoxy on MDO. Coating the paper with epoxy is just throwing away epoxy, all it needs is primer and paint.

The primer does a good job of filling the paper up rather than going directly to paint, as the primer has more body than paint does, and will give you a more uniform color coating after 2 coats than going directly to the paint.

Pipe, your marine fir supplier must suck, because we have far fewer issues than you are relating here. Much of it is coming from Roseburg over here, and is as good as any fir marine as I've come across. They also produce the MDO we have access to and it has been phenomenal.

http://roseburg.com/cgi-bin/s-mart.pl?command=showcatsub&sub=110_15

http://roseburg.com/cgi-bin/s-mart.pl?command=showcatsub&sub=110_10

They distribute all over the US, so not sure if that's who yours is being sourced through.

E

pipefitter
07-07-2012, 12:00 PM
Pipe, your marine fir supplier must suck, because we have far fewer issues than you are relating here. Much of it is coming from Roseburg over here, and is as good as any fir marine as I've come across. They also produce the MDO we have access to and it has been phenomenal. E

I didn't say it sucked at all. It may suck compared to some panels designated for boat building, such as Bruynzeel, Shelmarine, or panels which follow a true BS1088 rating, but I would use it and have some of it in my own build. Also, there has never been a fir panel made that does not come with checking present in the face veneers unless it is covered in MDO. I was using it nearly 35 years ago with up to 13, equal thickness plies in a piece of 3/4" and 7 in 3/8 and this was rated as "Marine Exterior", and was allowed more voids and plugs than designated boat building panels.

Lately, that which I have seen around here, in 5/8 and 3/4, has a thinner sanded face veneer that they add in with the ply count per sheet, even though that structurally, it really doesn't count with the final ply with regards to the grain orientation along it's length. Maybe yours doesn't have this thin veneer on the face and you may have to look hard for it and I don't expect MDO would for having the phenolic paper as the finish ply.

A lot of fir has been used in transoms and decks for years with FRP boats, and upholstery backers that weren't expected to outlive the upholstery itself. I used it for the sheer deck on my boat, and the upholstery forms for my seats, but they are all covered on the weather side with epoxy/glass, and my boat kept covered.

I've worked on boats my entire life in some fashion and replaced quite a few transoms and decks to better than factory for the most part, so I can understand the difference when you say that what you get is the best that "you" have ever used.

I am a fan of MDO, and have suggested it here on this forum a few times. In some cases, I think it better alternative to Okume or Meranti if weight is not a concern and for boats that are to be painted, or as an alternative to covering topsides with glass/epoxy.

pipefitter
07-07-2012, 12:27 PM
Spokaloo said:

The primer does a good job of filling the paper up rather than going directly to paint, as the primer has more body than paint does, and will give you a more uniform color coating after 2 coats than going directly to the paint.


That's the best description of Primer I've ever seen.
Now how about a something on the "Sealer" and it's relationship to "Primer".
We shouldn't need Sealer on MDO though, but on plain wood, we might eh?


Things may have changed, but Kilz, or other shellac based primers have always been superior to traditional alkyd primers over wood. It sticks to anything and is much more contaminate friendly and is a much better sealer.

csroy8014
07-07-2012, 12:55 PM
Would it be correct to fiberglass over the transum and the bottom of an MDO built LYS? what about glassing the butt joint on the sidepannels?

pipefitter
07-07-2012, 01:30 PM
Would it be correct to fiberglass over the transum and the bottom of an MDO built LYS? what about glassing the butt joint on the sidepannels?

If you are going for a seamless and smooth finish, most definitely. If you have to glass over a bottom, you could use a quality marine ply for the bottom and MDO for the sides, with the exposed edge grain of the MDO being covered with the bottom glass. You could also tape these joints, but by the time you add enough filler to make it appear flat, you are going to be 10-12" on either edge of the joint tape anyway. This is certainly acceptable on the butt joints, as most hulls are curved and easier to hide the tapes because of this, but on a transom, it would be best just to glass the whole thing to save on filler, and in that case, might just be better to use regular marine plywood.

I know of a builder who used to remove the paper of the MDO on either side of the butts to countersink the fiberglass tape, and to allow the epoxy to be bonded to raw wood. If I recall correctly, he used a router and did this before the panels were installed. This is how I would most likely do it myself when using it in a seamless, composite application.

csroy8014
07-07-2012, 01:53 PM
What type of resin and fiberglass would one use in the application?

Spokaloo
07-07-2012, 01:58 PM
Glass over MDO is just a waste of MDO. If you're going to glass, get regular ply and cover it with glass.


so I can understand the difference when you say that what you get is the best that "you" have ever used.

Condescending, but my point is that the '60s vintage boats I've worked on (in my not nearly as illustrious career as yours) the MDO has been of a similar core and construction as what Roseburg is producing lately. Granted the number of voids has gone up due to the loss of coastal old growth that they harvest, but it still has similar characteristics, MDO v. MDO.

E

pipefitter
07-07-2012, 02:34 PM
Glass over MDO is just a waste of MDO. If you're going to glass, get regular ply and cover it with glass.



Condescending, but my point is that the '60s vintage boats I've worked on (in my not nearly as illustrious career as yours) the MDO has been of a similar core and construction as what Roseburg is producing lately. Granted the number of voids has gone up due to the loss of coastal old growth that they harvest, but it still has similar characteristics, MDO v. MDO.

E

No, it's not meant to be condescending and I apologize for that. Just hard to relay in text. Plywood with a characteristic of voids being present, more so with fir panels, was always rated as "marine exterior", and this from a time when they had less voids. For example, I would not recommend using panels with voids with boats that live in the water at or below waterline, even periodically. When posting on these forums, you do not know who is reading, or who will take the information to heart and just because a product may have "marine" in it's designation. "Marine" does not always mean for boat building. Trailerable boats for day use are more forgiving.

It was actually fir marine ply that was greatly responsible for giving the notion of wood as being a bad thing in modern hulls. Now it's a major selling point for the other core materials used such as nidacore or the other foam or cell core materials, even though you can barely fasten anything to them mechanically or structurally. Now the advertisements proudly state; "No wood" as if that's a good thing. It may be great to all who never have to build anything on or around it.

If you are going to go for a slick finish over ply, MDO can save a lot of putty work if used strategically and treated or finished say like drywall joints on a house. The raised large grain of fir will indeed telegraph through glass and cost a ton of sandpaper and time on the long file. MDO, even when combined with fiberglass can save a lot of this if one must use fir. Otherwise, it's just better to cover regular fir ply with heavy glass and left as a commercial finish, which isn't always a bad thing.

pipefitter
07-07-2012, 02:48 PM
Here is a boat that is built with Fir Marine Exterior. At it's thinnest point, the glass covering may be a minimum of 3/16", quite unlike the thin 9-10 oz cloths typically suggested on most designer doll boats around these parts. In the event that the wood were to rot on these hulls, the glass could, in part, stand on it's own for quite awhile.

You can see the weave of the heavy cloth in this photo, and that what you can't see, is covered with a pretty heavy orange peel straight from the paint roller. It's not a bad finish for commercial work and I actually prefer it on these types of boats.

http://i99.photobucket.com/albums/l309/tigmaster/Netboat2.jpg

seo
07-08-2012, 09:01 AM
I have sealed MDO with Pettit acrylic sealer, and was surprised by the extent that it penetrated. A full sheet of MDO (two sided) would use about a pint of sealer, rolled out thin. This suggests that the suface of the MDO is porous to some degree. I can't say that I ever noticed a difference between the panels that I sealed like this, or those that I just painted. Both seemed to hold paint equally well.

xrayengineer
08-09-2012, 10:17 PM
1/2'' MDO= $39/ sheet. 1/2'' Fir Ply= $70/ sheet. Here in St. Augustine.

Fir is tough to paint. MDO is made to be painted.
Hey potomac, where do you get the mdo in st augustine?

Rigadog
06-12-2015, 09:03 PM
Anyone currently building a boat with MDO?

Reynard38
06-12-2015, 11:19 PM
I'm going to use it for some interior panels in the cabin. Anything that will get painted.

David G
06-13-2015, 01:31 AM
Anyone currently building a boat with MDO?

I used some on a small repair recently. Why do you ask?

chuckt
06-13-2015, 07:37 AM
Really heavy isn't it? I've read fastening is an issue but not sure why.

David G
06-13-2015, 08:21 AM
Offhand, I'd say the weight is comparable to meranti. Fastening into MDO shouldn't be be an issue, and more than into any plywood panel. Less than some.

Paul Schweiss
06-13-2015, 10:58 AM
Having repaired and re-painted several older Chris Sea Skiffs up to 42', along with Lyman's and Thompsons, they used MDO extensively. In a Production setting it saved a tremendous amount of finishing time as the Overlay provided such a great surface for paint. Most of the issues I saw were at the exposed edges of the laps where water had gotten in or abrasion had chewed things up, even where the fasteners had pierced the overlay, the putty they used gave a lasting finish. Providing you can find good plywood laminates, sealing the exposed edges with epoxy would give excellent results.
Years ago we could even order up factory scarfed panels, though handling them was not easy.

pcford
06-13-2015, 02:55 PM
Having repaired and re-painted several older Chris Sea Skiffs up to 42', along with Lyman's and Thompsons, they used MDO extensively. In a Production setting it saved a tremendous amount of finishing time as the Overlay provided such a great surface for paint. Most of the issues I saw were at the exposed edges of the laps where water had gotten in or abrasion had chewed things up, even where the fasteners had pierced the overlay, the putty they used gave a lasting finish. Providing you can find good plywood laminates, sealing the exposed edges with epoxy would give excellent results.
Years ago we could even order up factory scarfed panels, though handling them was not easy.

I will add this to Paul's comment...many years ago I replace the port shelf on a small Chris cruiser...probably in the late 70s or early 80s. (an aside...as I recall it was owned by a guy named Ballmer who was a programmer and lived on the east side. Brush with fame?) Anyways, the original shelf was stamped as as exterior grade doug fir. I put in a marine grade doug fir replacement. As I recall, the exterior grade member was better quality.
Chris Crafts are lightly built boats...but I have never seen a piece of chumpy wood in them. They bought quality materials. The point is that the MDO panels of today are unlikely to be the equivalent of those of 60 years ago.
Whether or not this will disappoint the boys here that are always wondering "Can I get away with...." is likely to be answered in the negative. Quality materials cost money.

David G
06-13-2015, 03:13 PM
Pat is correct in that - years ago, when I first started using MDO, pretty much any sheet you grabbed was going to be top-notch. Softwood panel grading, in general, has steadily laxened over time. Top-notch MDO is still available, but one has to look harder for it. I haven't had to buy any for over a year, just used up some old stock. Soon, though, I'm going to have to dig in and do some research, as my go-to supplier has changed the spec to allow less durable species as core veneers... <sigh>

Rigadog
06-14-2015, 07:47 AM
Really heavy isn't it? I've read fastening is an issue but not sure why.

I think you are thinking of MDF, very different stuff. I am thinking of doing the hull of Reuel Parker's Sea Bright 33 built lapstrake out of it. I would only sheath the bottom of the hull for abrasion and simply paint the rest. MDO plywood takes paint and glues very well by all accounts.

Rigadog
06-14-2015, 07:52 AM
Ollympic makes some very nice MDO ("Signal") and it's not cheap. I used some recently some and was impressed.

I am thinking of the savings in labor as well as not having to drape the entire hull with Xynole and then having saturate and sand the all that epoxy.

simmons22
11-18-2015, 03:36 PM
MDO ? Did someone say MDO ? I'm on my first build, the 22 foot Simmons Sea Skiff. I ran across MDO through a "discount-cash-only-craigslist warehouse". I bought some and used it on some projects and discovered it was as good or better than any sheet goods I had been using....and at a very sizable discount. For my build, I had already purchased several sheets of 3/8" marine ply, so I used it up on my bottom. Since I wasn't too keen on a 22 foot boat with only a 3/8" bottom, I doubled it, so it now has a highly respectable 3/4" bottom. So that pretty much used up my marine ply. So, I made another visit to my 'discount' guy and bought a number of sheets (3/8"; lots of 1/2"; some 3/4"; and a couple of thicker sheets). Once turned over, my boat got all of its side strakes out of 1/2" MDO....garboards included. I really laid on the epoxy on the garboards, but for the remaining strakes, only lightly epoxied the "inside" face (interior of boat, with no paper on it). All exterior faces of MDO are paper. All my cabin is MDO; all my hartop walls/roof are MDO. My 'splash rail' I'm adding is MDO... My floorboards are all MDO. I went totally MDO. It's very tough stuff. Paints like you wouldn't believe. Very few voids, if you can believe that, especially considering my source!! I'm sold on it. I was able to pick up a few sheets of two face paper, but most of mine is just one face. Sometimes the wood face has the typical "football" fillers; the other wood faces are just as "A" grade as one could ask for. MDO......perfect for boats! And with the current advent of "highway" HDO, it looks like boatbuilders delight....it's paintable. If I were to build another boat, it will be completely MDO'd. (well, other that maybe the trim....use oak, ash, mahogany, etc, but the body is all MDO).

Rigadog
02-09-2016, 11:17 AM
I'm ordering 5/8 MDO for the bulkheads of the Reuel Parker Seabright 33 I am starting. If the cores look good I will probably do the hull in the same. My conundrum then will be do I sheath the whole thing inn Xynole/epoxy or just the bottom and maybe garboards, or just coat it with epoxy and paint? Opinions?

Spokaloo
02-10-2016, 12:19 PM
If you're going to fiberglass, skip the MDO coating where the glass goes. Epoxy grabs wood much better than paper, and you're still only getting the adhesion of the paper to the wood as your bonding strength.

Single sided MDO with the wood out will give you a good glassing surface.

Otherwise, just do straight MDO, no glass, and a glued strip to protect the ply edge. Works like a champ.

E

oldsub86
02-13-2016, 11:00 AM
If you're going to fiberglass, skip the MDO coating where the glass goes. Epoxy grabs wood much better than paper, and you're still only getting the adhesion of the paper to the wood as your bonding strength.

Single sided MDO with the wood out will give you a good glassing surface.

Otherwise, just do straight MDO, no glass, and a glued strip to protect the ply edge. Works like a champ.

E


What sort of glue for the strip on the edge? PL Premium work well enough?

Spokaloo
02-13-2016, 11:46 AM
My LYS is my test bed for using PL exclusively in a boat. PL backed with screws, as it is a good gap filler, but can't be used like epoxy where glass reinforcement is used. The designer calls for Sikaflex, but I wanted to give PL a whirl.

The results speak for themselves.

E