sealing marine plywood

Greetings.  I need a recommendation on the best method / product to seal marine plywood.  I am having to replace the bottom of a 16 foot flat bottom river boat.  The boat is about 20 years old and has been compromized and starting soaking up water.  The original plywood was treated with "Cop-o-tox"  Then assembled.  After that several coats of Spar varnish were applied.

I would think advances have been made over the 20 years in products to use.  I would hope to get a general consensus on the right product (s) to use to get another twenty years out of the replaced bottom plywood.  Any help would be greatly appreciated. 

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RE: sealing marine plywood

Two coats of unthickened epoxy will do the job. Apply the second coat within 24 hours of the first. Better yet, apply a layer of fiberglass with the epoxy. Should last pretty much forever. -Wes

RE: sealing marine plywood

Thank you Wes.  That takes care of the bottom.  I am planning to apply one coat of epoxy and a layer of 6oz cloth.  Should I applay another coat of epoxy 24 hrs later?

In part of this project I have sanded the sides back down to the wood.  When I apply the cloth to the bottom, I plan to let it extend over the sides about about 2 inches with epoxy.  As that sets up, should I apply a thin layer of epoxy to the sides and then several coats of spar varnish in seperate steps.  I want to leave the wood exposed and not paint it.

Any advise would be greatly appreciated.

RE: sealing marine plywood

It depends on how smooth you want it. A coat of epoxy will seal the wood, but is not necessary unless it is very dry and porous. When it is dry to touch, spread the cloth and saturate it with epoxy, which will be absorbed by the cloth and stick to the bottom. Smooth it out with a plastic auto body squeegee. That will protect the wood but you will still see and probably feel the cloth weave. Another coat or two of epoxy will give you a smooth finish. Sanding will make it look opaque, but varnish will make it look clear. You can wait longer between coats but need to sand the epoxy to assure that the next coat adheres. Hope this helps. Anybody else have some advice? -Wes

RE: sealing marine plywood

  Cop-er-tox is an anti fouling bottom coating, Does this boat stay in the water? If so you may want to look into a bottom paint over the epoxy.

  My thoughts are to carry the fiberglass cloth up the sides to above the waterline at a minimum and if possible glass the entire side area. A lot of protection at a minimal cost.

  Another thought is how the new ply will be attached, is this a stitch and glue boat? Are you familiar with fillets and taping?  If the new marine ply is the 'big box store' grade you may want to apply fg cloth to both sides to prevent checking of the face veneer.

RE: sealing marine plywood

Thank you for the information.  I am planning to apply a sheet of 6oz fiberglass cloth from bow to stern, with about 6" overhang on all sides.  

The original plywood bottom was attached with a 2 part epoxy that I remember being purple.  That was about 20 years ago.  I was planning to use West marine epoxy this time to "glue the new plywood to the frame.  Tack it in place with a few nails then use 1 inch brass screws spaced about 6 to 8 inches apart.  After a couple of days of drying, curing, etc, then apply a coat of epoxy to the clean sanded bottom and sides.  Spread the 6 oz cloth over the wet epoxy, working it into the epoxy.  Let that dry for a day or two then another coat of epoxy.

The boat is not stained or painted and plan to leave it that way.  Over the years it has gotten 2 or 3 coats of spar varnish per year to keep the Wow factor.

The boat is trailered to and from the water so it does not stay wet.

 I would like to know if the steps I described above are reasonable and prudent.

Thanks again for the advice.  

RE: sealing marine plywood

I wouldn't put cloth on wet epoxy - let the epoxy kick, and while it's still "green", drape your cloth on and wet it out. Or don't bother with the first coat at all - just drape the boat with the cloth, wet out the cloth, then apply two more coats, each one after the previlus coat has kicked off but is stil green. Check out the section in "Shop Tips" in this website. 


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