The Rot Doctor


Subject: covering cheap plywood in boats
Date: Wed, 13 Jan 1999

Hi doc,

I'm sure glad you make housecalls. I'm building a 19.5 ft. Ohio sharpie from Rueul Parker's Sharpie Book. I live on Prince Edward Island (near Nova Scotia) and plan to trailer the boat.

Most things I've read say marine ply and white oak are best, and I'm sure they are, but I'm wondering if it would be totally foolhardy to build from spruce and exterior-grade ply. As you guessed, I'm trying to do it on the cheap. The boat's hard chine 1/4 and 3/8 ply and would use about ten 4'x8' sheets.

I'm on your side on the exterior vs marine ply issue. Marine ply is better, sure, because it has more plys and fewer voids. But, as you say, MUCH more expensive. I'd use the exterior ply -- if you can get the CPES (see below). Get the best exterior ply you can find -- no knots!

What I planned to do was soak all pieces with epoxy (CPES?) before assembly, glue and screw it together, sheathe in epoxy and cloth or least tape the seams, and paint the whole works to give it some UV protection.

This is the way to go, paying special attention to the edges of the ply after they have been cut for assembly. Flow all the CPES into the edges you can, and then give the surfaces a couple of coats. You'll get about 300 sq feet per gallon on surface application.

And you know what? You'd need to do this with the marine ply as well to get assured protection.

Your only problem will be getting the CPES into Canada. No US carrier will ship a Hazardous Material across the border. Canada has no problem -- it's the US carriers. The way Canadians have been getting their CPES is by having us ship it to a US location (friend or mail drop) and then they come across the border and pick it up. We supply all NAFTA documents to keep Canadian Customs happy.

And yep, the paint will do just fine as a UV protection, and will stick well to the CPES-treated wood.

My problem is that while some books don't totally rule out exterior grade (it's 1/8 the cost of marine here) they don't get specific about the trade-offs. I realize that it would have to be protected better than marine and this would drive up the cost, but I think that overall it would still be the cheapest way to go. Do you think I could get 20 years out of the boat using this approach?

Great web site.

Rob MacL.

Twenty years life for such a boat would be no problem if the paint was kept in reasonable condition. I would think the taped/epoxied seams would do just fine. It's the edges where the water and rot usually start on the ply -- and you'll have protected those.

Come back if you have more questions. Sounds like a fun boat. Had one like it when I was a kid, just a little smaller. No epoxy then, and it didn't last twenty years either!