My outboard engine sits on a support of plywood. What could I do to make the plywood last longer.?

You simply need to laminate it with a coat of boat epoxy.

I have 4' x 8' plywood runners laying flat on the ground, I use as a sidewalk around a barn I use for building boats. . . that plywood has been there for 10 years. No and then, I lean them up, wash them off with a power washer, and they look as good as new. . . and all I did to them was give 'em a good coat of boat epoxy.

You do that, and I guarantee you that piece of plywood will long out last you, and probably your grand kids.

John

Not too much, really. Even marine grade plywood isn't intended to get very wet -- the chemicals which are pressure cooked into it are water soluble, and they will leach out over time in a wet environment. The ideal thing to do would be to have the plywood floor covered with a sheet of metal, or replace it with heavier metal. If treating the wood is your plan, you can get really aggressive, and paint the plywood with "bottom paint" from a sailboat supply store -- the stuff's deadly, use caution. It's intended to minimize marine growth (algae, barnacles, etc) below the waterline on boats which are left in the water instead of being trailered onto dry land (tankers, larger sailboats, etc). If this is too much, I like epoxy enamel. It will buy you some time, but keep an eye on the wood for signs of softness -- you've got a lot of money sitting on it.

s d

I like lima's answer. be sure to spend the extra money on the proper materials and finishes. They're cheaper than a new motor should your transom rot underneath the finish and snap off. Do it once, do it right.

Remove any existing finish by scraping, stripping and sanding. Seal the exposed edges with polyester or vinylester resin. Prime the plywood with an epoxy primer such as Pettits' Easypoxy. Sand and finish coat with marine epoxy or enamel paint.

Treat the transom with a clear wood preservative. Let it dry thoroughly. Then use a marine paint followed by yacht varnish. The 'International' brand of paint do marine versions. You may have to visit a yacht chandler to buy the paint, but the yacht varnish is in most DIY stores.