MDO Board vs. Marine Ply

[Buzz73]

I am considering building a small flat-bottom skiff which requires 1/2" ply for the bottom panel. I have access to some 1/2" MDO and would like to know if it would be suitable for a hull bottom. I intend to fiberglass the outside of the hull, but to only epoxy-seal the interior.

Would the MDO work? Or should I stay specifically with marine ply?

[Rick Tyler]

I would love to hear what experience others have had. But.

I wouldn't apply fiberglass to the paper face of MDO. I would be nervous about the bond between the paper and the ply.

My other concern is that the MDO I have bought lately has had the same voids as regular sanded-face construction plywood.

[T.A.R.]

I have had success with it in making cabin sides for my schooner. Laminating two half inch pieces I found it worked well and finished very well. Sorry I can't provide info on glassing. The stuff worked well for the sides of a Quahog skiff though.

[merlinron]

do a search here, there are some threads about using it for boats. the replys are varied, and there are some warnings about the core quality( as there are with any plywood) but generally, i think they say it's ok to use. you just have watch what you buy, according to the replies.
i would also be concerned about epoxy bonding to such a slick surface. maybe a very light sanding would in order.

[ssor]

Not all MDO plywood is created equal. That which is made for highway signs is very good and that made for concrete forms is excellant but may be oiled. As for fiberglassing the paper side --try it on a piece and test to distruction.

[THX712517]

I built a 14' 6" flatiron sailing skiff using 3/8" MDO for the bottom and 1/4" regular ply for the sides. The chines are fiberglassed on the outside only. I haven't launched her yet but it looks nice and solid. I'll find out what's going on when I find a trailer and a hitch so I can launch her.

[Paulyboy]

You should also be able to find MDO faced with paper on both sides. Scuff the paper facing with fine paper before coating with glass. If you suspect any oil contamination, try dewaxing and thoroughly drying before glassing. I'm plannig on building a teardrop trailer with the stuff after i'm done wif my boat.

[bainbridgeisland]

Why fiberglass over MDO? The surface treatment of MDO stops checking as well as a thin layer of fiberglass does. Certainly, it is not needed to create a painted surface or prevent checking. The surface is ready to paint without any prep other than vacuuming (unless contaminated in your shop) and takes paint beautifully. You do not need many layers of sanding primer to fill the grain and can get a great, high gloss, paint job over one light primer coat. (By-the-way, the only reason to fiberglass MDO is for local impact or abrasion resistance.)

Why would anyone think that the surface of MDO would not bond to fiberglass? The paper-based surface is glued to the timber with the same glue that attaches the veneers together, so it is well attached. The surface is specially formulated to take paint. It is not at all slick. Instead, it has reasonably high surface energy (promotes wetting) and a fine textured high profile surface, perfect for epoxy or polyester resin to bond to. If you need localized abrasion resistance or impact resistance, you will find fiberglass bonds very well to MDO.

Using MDO for boats is nothing new. One brand, specifically targeted for boatbuilding, 'Super Harborboard' (hope I got the spelling right) has been around since at least the early 1950's.

[bluefish22]

I had the same question and tested fiberglass/epoxy on MDO. When I tore the fiberglass/epoxy away from the test piece, the MDO surface came away with the cloth and a fair amount of wood grain came away with the MDO surface. Based on the results I intend to glass/epoxy over my MDO bottom. I also tested lumber epoxied to MDO with the same result. For you PL Premium fans, I tested lumber glued to MDO with PL; same result.

[HarryH]

The MDO found at local yards is at least the equal of, and in many cases superior to the fir marine ply they sell, in terms of voids. The glue is the same, the number of laminars the same, but "marine ply" has suffered quite a bit in quality, it seems.

I would not shy from MDO....at least what I've seen around the Cape Cod yards, particularly as it compares to marine ply. While the size of voids along the edges of plywood is no guarantor of the inside of the entire sheet, it can be a good indicator....and boy, does MDO paint up well.

Look around, pick over the MDO stock.

My .02

HarryH

[seo]

I used quite a bit of MDO for bulkheads and deckhouse sides on a 63' wood passenger boat. It worked very well. Over the six years I owned the boat the quality of available material diminished: more voids, thicker interior plies, etc.
I did glue some with epoxy, right on top of the MDO. (A bit of trivia: MDO stands for "medium density overlay" which is the paper/resin composite that makes the face. Basically (I think) the same stuff as formica, and not too distant from masonite.
I've also used masonite a lot, and like it okay.
I don't think any of these materials are intrinsically "oily." The thing that seems to limit their gluability (is that a word) is how smooth and hard they are: little chance for glue to key to it.
seo

[Cuyahoga Chuck]

Laminating anything over paper suggests to me the bond at that point is only as strong as the paper.
On S&G designs where the ply/'glass lamination is done for structural reasons the interposition of a layer of paper may imperile the strength of the hull.

Charlie

[Rick Tyler]

Originally posted by Cuyahoga Chuck:
Laminating anything over paper suggests to me the bond at that point is only as strong as the paper.This is what I was worrying about. I've seen two pieces of wood glued together with a sheet of paper in between them so that they could be shaped together and then easily snapped apart with a chisel. It doesn't give me much faith in any two surfaces glued together with paper in between.

[bainbridgeisland]

Originally posted by Rick Tyler:
</font><blockquote>quote:</font><hr />Originally posted by Cuyahoga Chuck:
Laminating anything over paper suggests to me the bond at that point is only as strong as the paper.This is what I was worrying about. I've seen two pieces of wood glued together with a sheet of paper in between them so that they could be shaped together and then easily snapped apart with a chisel. It doesn't give me much faith in any two surfaces glued together with paper in between.</font>[/QUOTE]I have seen MDO that has been fiber glassed on a number of occasions. It always seemed sound. As stated before, it serves no purpose to glass MDO so use an alternative material. If you are worried about the MDO surface, test MDO against plywood. Make up a sample of each and then destroy them. You will be able to tell if the MDO surface causes a problem. I can tell you from personal experience that both epoxy glue and 3M5200 makes up perfectly adequate joints on MDO surface.

I use MDO and have built entire boats out of it without problems. However it is an inferior material to marine plywood. So it is not my first choice. You have to consider the service though. For example, painted MDO has better rot resistance than painted okoume plywood. However, I think it is a toss-up when comparing rot resistance of painted MDO and epoxy coated okoume plywood (assuming the edges are sealed on all). And then, epoxy coated, fir marine plywood has the most rot resistance of the three.

[ 12-01-2005, 11:21 PM: Message edited by: bainbridgeisland ]

[Ron Williamson]

Everyone seems to have a problem with the "paper" aspect of MDO.
Paper is wood fibre with glue,not unlike the epoxy fillets that stick boats together.
Countertop laminate is paper stuck together with waterproof glue.
If you have a critical structural joint,with a small surface area,it might be a good idea to remove the paper,but it is a very good idea to backup the glue with fastenings anyway.
R

[Lewisboats]

Most of the boats that I have heard about being built with MDO used the paper side on the inside, giving a nice surface to finish for the interior, the exterior gets the unfinished side to be covered with 'glass.

Steve

[bainbridgeisland]

Originally posted by Lewisboats:
Most of the boats that I have heard about being built with MDO used the paper side on the inside, giving a nice surface to finish for the interior, the exterior gets the unfinished side to be covered with 'glass.

SteveOne version of MDO marketed specifically for the marine industry used to be called 'super harborboard' or 'super harboard'. I saw it advertised in the late 50's and early 60's. When Aprenticing in the 70's, some of the commercial boats I worked on used it for superstructure, transom and bulkheads. They had been in service at least 10-years and seemed sound. These surfaces were not fiber glassed, just painted. They were easy to maintain. They did not need paint as often as the hulls which were touched up as needed and then got a new coat of paint every 3-years or so.

I have seen rotting hulks with MDO aboard. It seems to survive quite well compared to the rest of the structure with the exception of the edges. So if you choose to use it, be sure to seal the edges of the plywood.

[maa. melee]

Is Super Hardbord still around?? Who distributes it?? I've read about the stuff in old Popular Mechanics magazines and in old boat plans but haven't seen the stuff. Can anyone supply me with any information on this stuff

[maa. melee]

bump for super harbord