Waterproofing plywood using epoxy paint

It is my first time building or repairing a boat of any kind, and I plan to build an 8' by 2' boat using treated plywood. I have no experience with this and welcome any advice on what would be best to seal and waterproof the boat, I was looking at using CPES but am unsure.

 

Welcome to the forum.

Current treated plywood in the USA isn't compatible with epoxy coatings. The recent change in chemistry makes this, though you can get CCA treated plywood (the old stuff) if you have a commercial license.

CPES is more of a marketing tool more than anything else. It's not water proof (not even close) and has limited uses, frankly.

If you do get some CCA treated plywood, you can use regular epoxy and get good results, assuming the stock is dry (12% moisture content or less).

Epoxy and epoxy paint aren't the same thing, though their chemistry have some similarities. Neither epoxy paint, nor CEPS will seal wood. 100% solids, marine grade epoxy will. Several brands (types) of paint actually seal wood better than CEPS.

An 8'x2' boat can't support much. Assuming a real 8'x2' box, without curves, just a box, a 2" draft will yield 166 pounds of displacement. At 3" of draft you'll get about 249 pounds of displacement. Assuming you'll put some bottom rocker for maneuverability and shape in the ends, you'll decrease these figure substantially.

What design are you building?

 

What would you recommend I use to waterproof the plywood?

 

Why do you need to waterproof the plywood?

 

why use treated plywood? If you are going to store the boat out of the water there is no reason to use treated wood. It is toxic and costly. If this is your first time build I suggest just using exterior grade doug-fir AC plywood, nicer finish and it will cost less. Doug fir is fairly rot resistant by itself and if stored out of the water will give you many years of service.

Just use gloss exterior porch paint, seven layers after it is built. Oil based paint is more durable, but takes much longer to cure between coats, for a boat like this Latex paint will work fine. And that will offer a fair amount of protection to the wood.

 

Thank you very much Petros

 

A few things here - you cannot paint or epoxy coat treated lumber for about 6 months - to allow the 'treating' chemicals to 'dry out'

The difference between epoxy paint and marine epoxy is that that marine epoxy is raw resins and curing agents and a few misc additives. The epoxy paints have pigments and thixotrophic agents. Except floor epoxies which skip the thixotrophic additives for better self leveling.

The product metioned above is actually 69% solvent and 31% low end epoxy. Solvents help the epoxy 'soak in' - the solvents soak in and carry the epoxy. The question here is that solvents are cheap in bulk so are you getting ripped off by something with so little epoxy in it?

You can seal wood quite well with epoxy - you always need two coats (1 coat cuts water take up approx 50% - two coats about 90% according to my personal tests).

Note that wood expands and contracts with water content and epoxies and other things with temp. So they move differently. So you want a flexible epoxy that will soak in a little bit for max bonding. Fortunately solvents are also flex agents (by messing up the epoxy cross linking). Solvents also add pot life time and thin the epoxy making it easier to apply.

So best plan for you is to wait a few months. Then one coat of thinned epoxy (marine or epoxy paint). Then at least one coat more of epoxy - thinned or unthinned.


If you don't need the natural wood finish, you can replace the two coats of epoxy with the LPU aluminum filled aluthane paint (PAR has test sample of this LPU coating). The aluminum flake pigments stack on top of each other as the coating shrinks when the high amount of solvents flask off. This produces a 'metallic' barrier and works as well as two coats of epoxy in sealing out water.


paul oman
progressive epoxy polymers inc

www.epoxyfacts.com

 

I've found the new CA chemical process on PT, doesn't permit epoxy to get a good "grab" on the surface of PT lumber or plywood. The old CCA chemistry did permit epoxy to get a good bond with the surface. The old CCA treat stock is available and used commercially, but not sold at the retail level.

If you want to use CA treated stock, I'd skip the epoxy all together and just go with Paul's Aluminuthane (did I spell that right) product, which can be over painted the color you want (it's silver out of the can). It'll work very well as a sealer, though not a completely water proof barrier, good enough on a PT plywood built boat, as you've described.

Lastly, as Paul has mentioned and I'm sure you know, if you've ever worked with PT materials, it needs some time to dry out. I've found stickering it up in a "Mini Storage" unit, can make quick work of this, if you put a box fan on it and let the heat of the day and the closed door cook off the moisture. Another method is a regular metal storage shed, with a black painted roof and no insulation can be effective, if in the sun most of the day.

 

How about ths?
If your planning a "Sneak Box" or small "Jon", how about 8" planks of whatever the sawmills locally can come up with, that is without knots or swirls n the Grain.
Then use Battens to make it sorta water proof.

With a boat that small you don't have to worry about a little water getting in the boat.
And where you find that 'leak' you can stop it with a little creativity.

 

I've been told that epoxies (or anything else) with solvents/vocs (vs 100% solids epoxy) leave lots of tiny holes as the solvents evaporate. So while it looks waterproof, it is actually porous.

 

I'm curious about all these opinions. I think I'll contact a Mfg. and ask for their opinion.
Does anybody have a Mfg they'd like to hear from?

 

Hey, Thud, they're not really manufactures, so much as reformulators, at least in regard to epoxy. The furthest you have to look is at post number seven (above). Paul Oman has been formulating epoxy for over 20 years.

 

very very few epoxies (especially those used in marine use) contain solvents. -- except when thinned to prime and seal (self made or pre-made). Don;t know about the 'holes' seems unlikely, but in any event you need at least 2 coats of any epoxy or non epoxy product to seal. One coat leaves pin holes, thin spots, missed spots etc. Multi coating pretty much fixes these issues.

 

What about plain ol' Oil based House paint for a boat made with Boards?
It might work on Plywood too, but planks are just plywood...

 

Alkyd paint works fine, but it's not water proof, nor especially hard and durable.