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How do I repaint my trailersailer’s bottom?

2021 June 1

Dear Boat Doctor,

I have a 1982 S2 5.5 Grand Slam. It’s stored on a trailer and has the original hull paint. I want to paint the bottom of it to freshen it up and to cover up the very fine spider cracking. What would my process be, and what type of product would you recommend?

Rich Safranek

Kennesaw, Georgia

Dear Rich,

Painting your bottom should be pretty simple, but getting access to everything can be the hard part. It would be ideal if you could suspend the boat to get access to the full bottom and centerboard. 

Start by masking off the line between the gelcoat and bottom paint with high-quality blue painter’s tape. Use ¾-inch tape to contour to the water line. Add another layer, overlapped by half, to give a 1½-inch margin of protection. This tape is used to protect the gelcoat. It will get a little beat up so it will be replaced before painting.

The next part is getting all the old paint off, which will be a big messy job. I would suggest sanding with 80-grit on a random orbital sander. Sanding bottom paint is dusty, and the dust is toxic, so do your best to control the dust. A vacuum on your sander helps, and a good dust mask is imperative. Once you get all the paint off, remove the masking tape and wash the hull well and scrub with a Scotchbrite pad. Allow the hull to dry overnight and then wipe down with acetone.

With a clean hull, you can start building your foundation. First, replace the masking tape with a fresh layer, double up, so you have a wide margin. I recommend starting off with Interlux Interprotect 2000E. This product is designed as a barrier coat to help prevent fiberglass from water absorption. Interprotect is a reasonably thick primer and may fill your spider cracks, but if not, you can fix them in the next step. Use a quarter-inch nap roller to get a smooth finish.

The consistent coat of Interprotect will show any defects in the fiberglass. You can fill any scratches or cracks with 3M Premium Filler, which is an easy to use, fast-dry filler. Fill any defects and lightly sand the bottom with 80 grit. Next, we’ll roll on another coat of Interprotect to seal off your repairs and to give a good surface for the paint.

Interprotect cures so hard and smooth, that it’s tough for the paint to stick to it. Combat this by applying the first coat of paint when the last coat of Interprotect is “thumbprint tacky.” Thumbprint tacky means you can leave a thumbprint in the primer, but the paint doesn’t stick to your thumb.

I think Interlux VC17 would be great paint for your boat. Roll on two to three thin coats with a quarter-inch nap roller.

Your new bottom paint should last for many years of trailersailing. If the paint starts to look thin or scuffed up, you can add another coat. I’d scrub well with a Scotchbrite pad and roll on another coat.

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