How to Waterproof Plywood for a Boat? – 2 Methods

This is recommended for sealing plywood boat floor, but not if you leave your vessel in the water often.

If you’re alright with that, then go ahead and follow these steps.

1. Clean and sand the wood.

Use any cleaning cloth you have on hand that’s suitable for wood to prepare the hull. Dab it in acetone to get it dry and get rid of any dirt and other unwanted specks the boat may have.

Follow the instructions given by the acetone manufacturer on how long you’re supposed to wait for it to dry. Once it’s nice and dry, you can begin sanding.

Use sandpaper that’s at least 80-grit, since it’s what’s ideal for most finishing endeavors. Shoot for as smooth a surface as possible. And don’t forget to wear your mask!

2. Apply the sealant on the seams then sand it again.

When it comes to sealing plywood for marine use on the cheap, I always use a marine sealant like the ones offered by Loctite or other relevant brands. This prevents the common issue of the plywood’s edges soaking up water.

Once that’s done, you can now start applying the sealant. Follow brand instructions on how to use the caulking gun, but it should be pretty easy to aim toward the wood.

All locked in? Now, the waiting game starts. Most sealants take a whole day or two to completely cure.

After the sealant has cured, you need to sand its surface to make sure that the paint will stick. Don’t forget to do that before applying the paint’s primer!

3. Do the first coat of primer then sand.

Check the bottle on how best to apply the primer. You can probably use either a brush or roller, whichever you’re comfortable with. I suggest you stick to the recommendations as to how thin or thick you need to apply it.

Wait for it to dry. It shouldn’t take more than an hour if the temperature and humidity in your work area are cooperating.

All dried up? Time for another round of sanding and cleaning with the tack cloth.

4. Apply a second primer coat before brushing or rolling the latex paint.

You basically repeat the steps for applying the primer once you’re done prepping the wood for painting. Again, wait for it to dry.

From here on, you have free rein on how you want to paint the plywood. Just be sure to read the label! The primer should help in getting a smooth finish, but be sure to take care of imperfections.

At best, latex is the only waterproofing paint for plywood that does a decent job if epoxy is out of the equation. It may have a few major downsides, but I can’t name a better option when painting marine plywood.

This method is relatively harder, as a single mistake can cause you to start over. A helping hand or two would be most welcome, too.

1. Sand and clean the boat then lay the fiberglass cloth.

Put on the mask, then sand the hull’s exterior with 60-grit paper first. Swap to the 120-grit and 180-grit as you continue to do so for a smoother result.

Wipe the surface down with a cloth, and dab it with acetone if needed. Just make sure that it’s dust- and debris-free before laying down the cloth.

Afterward, roll the fiberglass cloth carefully to cover the entire surface. Naturally, it will be curved, so you can expect some wrinkling. Use the brush to push down and smoothen the cloth.

2. Mix the epoxy then apply the first coat.

It’s time to waterproof plywood subfloor. With epoxy resin, you always need to stick to the manufacturer’s recommendation for the ratio.

Stir both the inside and outside of the mix. Moreover, refrain from making one large mixture, as you may not follow the correct ratio!

Done with the mix? Start applying it by pouring small amounts on the fiberglass and using the squeegee to help spread it around for the mat to soak evenly. Flatten any excess epoxy you see.

You’ll need to cover the entire span of the hull. Once you reach the edges, feel free to cut off the excess cloth.

3. Wait for the coat to settle then apply the next one.

This will take just a few hours of waiting, more or less depending on the epoxy you used. You can apply the second coat even if the first one’s still a bit tacky.

This time, use the roller. Again, pour some epoxy over the surface then roll it on for good coverage while making sure there are no bumps.

Wait for this coat to dry then apply the third one and any subsequent coats you decide to apply, then wait for them to fully cure. Four coats normally get the job done for me.

The steps are essentially the same for the inside of the hull and when waterproofing plywood boat deck. So just repeat them once you’ve flipped your boat.