Hello! Making a new motor mount pad for my Catalina 22. I have two layers of marine plywood that I will use. I say all I need is Spar Varnish to keep the water out. My brother seems to think I need Penetrating Epoxy(P.E.) before I varnish. I counter that P.E. is for wood that has previous water damage. Your thoughts?
Is Penetrating Epoxy overkill?
- Thread starter gtg
- Start date
Let's at look the issues.
Applying epoxy to bare wood prior to varnishing seals the wood and prevents water from being absorbed into the wood thereby causing it to rot. That's the theory and it works so long as the epoxy remains protected by a UV blocking varnish and the wood is very dry before applying the epoxy.
However, (there are always howevers) if the wood is damp or if the varnish wears or chips the epoxy will fail. Damp wood will bake in the sun and the water in the wood will push against the epoxy lifting it off the wood. Basically the same thing that happens with varnish. If the varnish fails or in the places it does fail (and it will) the UV exposure will cause the epoxy to deteriorate. So if the wood is epoxied first maintaining the varnish is essential.
The use of "penetrating" epoxy is controversial at best. Epoxies marketed as penetrating epoxies are thin epoxies that are able to saturate the wood a little better than standard epoxies. How the epoxies are thinned makes a difference. Just adding a thinning agent doesn't really work because the thinning agent just flashes off and you end up with regular epoxy. One technique that works is to warm the epoxy prior to mixing with the hardener and warm the wood with a hot air gun. Mix the epoxy and spread quickly. The warm epoxy will be thin and will soak into the wood better than colder thick epoxy. Warming the wood first dries it out and reduces the amount of air and vapor locked in the wood fibers. After applying the epoxy, the wood cools and draws the epoxy into the wood. Putting warm epoxy on cold wood causes off gassing as the air and water vapor try to escape from the wood preventing the epoxy from saturating the wood.
If I was doing this project I'd just use thickened West System 105 to bond the 2 boards together and West 105 with the special clear hardener to coat it before applying several coats of a polyurethane with UV inhibitors. The special hardener will prevent the epoxy from turning milky white. Be sure to remove the amine blush before applying the polyurethane.
Jamestown Distributor's house brand, Total Boat, have equivalent epoxies at a slightly lower cost than West.
To settle the fraternal dispute, both of you are right and wrong. Epoxy before varnish is worth doing, it doesn't need to be penetrating epoxy.
Applying epoxy to bare wood prior to varnishing seals the wood and prevents water from being absorbed into the wood thereby causing it to rot. That's the theory and it works so long as the epoxy remains protected by a UV blocking varnish and the wood is very dry before applying the epoxy.
However, (there are always howevers) if the wood is damp or if the varnish wears or chips the epoxy will fail. Damp wood will bake in the sun and the water in the wood will push against the epoxy lifting it off the wood. Basically the same thing that happens with varnish. If the varnish fails or in the places it does fail (and it will) the UV exposure will cause the epoxy to deteriorate. So if the wood is epoxied first maintaining the varnish is essential.
The use of "penetrating" epoxy is controversial at best. Epoxies marketed as penetrating epoxies are thin epoxies that are able to saturate the wood a little better than standard epoxies. How the epoxies are thinned makes a difference. Just adding a thinning agent doesn't really work because the thinning agent just flashes off and you end up with regular epoxy. One technique that works is to warm the epoxy prior to mixing with the hardener and warm the wood with a hot air gun. Mix the epoxy and spread quickly. The warm epoxy will be thin and will soak into the wood better than colder thick epoxy. Warming the wood first dries it out and reduces the amount of air and vapor locked in the wood fibers. After applying the epoxy, the wood cools and draws the epoxy into the wood. Putting warm epoxy on cold wood causes off gassing as the air and water vapor try to escape from the wood preventing the epoxy from saturating the wood.
If I was doing this project I'd just use thickened West System 105 to bond the 2 boards together and West 105 with the special clear hardener to coat it before applying several coats of a polyurethane with UV inhibitors. The special hardener will prevent the epoxy from turning milky white. Be sure to remove the amine blush before applying the polyurethane.
Jamestown Distributor's house brand, Total Boat, have equivalent epoxies at a slightly lower cost than West.
To settle the fraternal dispute, both of you are right and wrong. Epoxy before varnish is worth doing, it doesn't need to be penetrating epoxy.
There are people who disagree that penetrating epoxy is all that good for whatever function. You could encapsulate the plywood well with regular epoxy in an attempt to prevent wood movement due to changing moisture content, and this would help keep your varnish over the epoxy from popping. There are others who have had experience that the wood under the epoxy can still rot eventually. I've seen other people (like this guy on YouTube who makes knives) use a vacuum pump to draw casting epoxy into the wood, bakes in an oven to cure, then he uses that epoxy impregnated wood for handles. That would probably be the most effective way to get epoxy to penetrate the wood. I guess the benefit of putting on 2 or 3 coats of plain old West Systems or similar epoxy, and then NOT sanding through, prior to varnishing, would be that you might not have to wood down the varnish as soon provided you keep applying a coat or 2 each season.
