Treating interior plywood in a small boat

[iansan5653]

Hello, I've got a 21' sailboat that's missing the covers for the storage under the settee berths and v-berth. I have the cushions but when you sit on them you just fall through .

Simple question - if I use marine plywood to replace the covers, do I need to treat it or should it be fine untreated since it won't be exposed to the elements? If I treat it, how should I do it - is epoxy around the edges enough, should I epoxy the whole thing, or should I paint it?

Thanks!

[Nicholson58]

Almost any finish is OK for interior, never-wet wood. Epoxy will be the most durable but any pretty good spar varnish is acceptable. Epoxy will give a very fast build.

[BCCLover]

We used regular household oil-based primer and paint for sealing/coating marine plywood in spots like that. We prefer white. Rustoleum is easily found in the US; Home Depot's Behr brand (oil-based enamel) goes on better in our experience.

[Sand crab]

+1 on spar varnish. The spray can stuff is pretty good. I like epoxy but its a PIA. Messy, expensive, drips, runs etc.

[iansan5653]

Cool, thanks! I have some epoxy but I kind of wanted to avoid it because of the mess. I'll check out spar varnish or I may already have a can of paint lying around I could use.

[Nicholson58]

Min Wax and Rustoleum both sell a decent spar varnish. Spray cans are crazy expensive and contain very little usable net material.

[evm1024]

Originally Posted by Nicholson58

Min Wax and Rustoleum both sell a decent spar varnish. Spray cans are crazy expensive and contain very little usable net material.

My local chain store (Fred Meyers) has Minwax Spar Varnish cans for less that $10 USD. I usually grab a few cans.

I use the quart size Minwax spar varnish for most interior jobs. For 'Hidden" jobs I'll foam brush on a few coats with light sanding to get a protective seal.

If the job is visible and I want it to look good I'll take more time with the brushed coats and sanding. Then this is where the spray cans come in. The last 2 coats are sprayed on for a very nice finish.

And of course spray for some touchup when needed.

[Cheechako]

Originally Posted by iansan5653

Hello, I've got a 21' sailboat that's missing the covers for the storage under the settee berths and v-berth. I have the cushions but when you sit on them you just fall through .

Simple question - if I use marine plywood to replace the covers, do I need to treat it or should it be fine untreated since it won't be exposed to the elements? If I treat it, how should I do it - is epoxy around the edges enough, should I epoxy the whole thing, or should I paint it?

Thanks!

You can just use exterior ply if you want. It has the same glue as Marine ply. The difference is Marine ply has no voids. Exterior is allowed to have some voids. But yes you can use interior, I would coat it with something. It may fail eventually, but will last quite a while.
Most paints last longer than varnish, I would use good oil base/acrylic paint for those boards. However, inside, again... varnish would be fine. I far prefer poly like Varathane inside. It's harder than spar varnish which is formulated to be softer and flexy...
You dont need to epoxy, it's a lot of trouble and inside doesn't need it. You must sand the gummy epoxy before putting a finish on it.

[garyfdl]

Originally Posted by Cheechako

You can just use exterior ply if you want. It has the same glue as Marine ply. The difference is Marine ply has no voids. Exterior is allowed to have some voids. But yes you can use interior, I would coat it with something. It may fail eventually, but will last quite a while.
Most paints last longer than varnish, I would use good oil base/acrylic paint for those boards. However, inside, again... varnish would be fine. I far prefer poly like Varathane inside. It's harder than spar varnish which is formulated to be softer and flexy...
You dont need to epoxy, it's a lot of trouble and inside doesn't need it. You must sand the gummy epoxy before putting a finish on it.

I'll second Cheechako's post, and don't recommend a spray. Use a brush and apply at least a couple coats. I'd paint instead of varnishing. No one's going to see them and paint affords better wear and protection (IMO). Makes sure you sand the edges round so they're not 'sharp'. I'd also go as far as plugging any edge voids with wood putty and do a good job sealing the edges up with what ever you use.

[s/v Jedi]

I recommend the TotalBoat Penetrating Epoxy. Thin it 50% with denatured alcohol and brush it on. Very quick and easy. Sand with 330 grit after cure. It will feel like a rock hard plastic but still look like wood. If it lives underneath cushions it’s ready but you can paint it as well.

The reason for my preference is that epoxy kicks after which it’s hard. Varnish takes too long and is too soft.

You can also use an epoxy barrier coat to get a white or grey finish. Try thinning on a test piece.

[thunderhoof]

You might consider Star Board or equivalent. It is a plastic material that can be used to replace marine plywood. It requires no finish and is completely impervious to the marine environment. Considering the price of wood products today it might even be cost competitive. It can be worked with normal wood working tools.

[chasmains]

I realized recently that there are a couple of wood rot fix products now. Rustoleum makes one and there may be others. These are interesting because they provide high penetrating epoxy. It acts like penetrating oil and goes rapidly into the wood grain and then cures. It leaves a sealed epoxy exterior. I was amazed when I tried it on some dry rot in my shed. Then I painted it onto the unpainted side of the shed doors and was again amazed at how it sealed AND hardened. No more rot in the last couple of years. What this means to me is that a treated plywood plate, maybe moist from being inside a boat, will resist for much longer than regular untreated marine plywood. And it has a hard surface to resist wear and damage better. I expect this may be a boat owner’s next best friend. Have a look. Curing time in a couple of hours is what I saw.

[Ramona]

I'm a heavy user of oil based paving paint. The stuff you paint on concrete. It's hard, no primer, just paint two coats. Does not chip and is easily overcoated.

[Boatwright]

Nothing wrong with exterior rated regular ply for a non-structural application like seats.

I've been using Rustoleum enamel for interior boat paint for decades. It is a high-quality alkyd (not the same as acrylic) oil-based enamel, reasonably priced and available everywhere. Lt. grey is the best choice - covers better, hides the inevitable dirt, and is the classic color for boat bilges, and hidden wood such as your seats. Follow the manufacturer's instructions re thinner and thin about 10% for good penetration. You do not need multiple heavy coats - one on the underside and two on top is plenty. The idea is to seal and stabilize the grain of the wood, while still allowing the wood to breath. Brush application is fine.

Regular epoxy resin is a bad idea because it traps moisture. 30 years ago dozens of mahogany runabouts (Chris-Crafts) were restored using West System epoxy as a sealer which was supposed to permanently protect the wood. It didn't and sadly many of those boats quickly rotted out. There is one epoxy product: Smith's Penetrating Epoxy Sealer that is very thin and works well as a sealer esp. under fancy varnished brightwork, but it is very expensive and very toxic.

Keep it simple. You can spend the money you save on rigging, etc.. Have fun and a welcome from one of those who find nothing quite as fun as messing around in boats.

[Pauls]

Lots of good info in the above. Use exterior ply. There is no advantage in the use of marine ply for this application.

An important point is that by far the most important area to seal on any plywood is the end grain, ie the edges of the panel. This will soak up a lot of whatever you use. Best practice is to apply a heavy first coat on the edges with a thinned paint, varnish, whatever. This will wick into the grain. Let it dry and then apply a second coat. It may take 3 coats to seal the edges. This is where the wood is most exposed to moisture caused expansion/shrinkage and to rot.