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How do I get rid of bottom paint buildup?

2021 July 1

Dear Boat Doctor,

I have quite a buildup of paint on the bottom of my boat. In some places, it’s worn away, in other areas, it’s thick and peeling. I want to get it all off and start with a fresh coat. Where should I start?

Pete Berens

Baltimore, Maryland

Dear Pete,

Bottom paint has a way of building up. We roll on a coat every couple of years trying to avoid having to do a real bottom job for as long as possible. I think your approach is the right one.

First off, bottom paint is toxic. It is not good for you or the environment. If you are going to set it free from your bottom, you need to be responsible with the waste. I’d also start with a discussion with your boatyard to understand their preferences or regulations. If you do take on  this job, you need personal protection equipment to protect your skin, eyes and lungs.

The simplest way to remove bottom paint is by sanding with 80 grit but you need to contain the dust. A vacuum on your sander helps, but you may need to tent your boat or put a tarp down to collect the dust.

A slightly less dusty method is to scrape. Use a carbide paint scraper, not a putty knife. A scraper, carefully used, will leave a very clean surface that you can clean up with a light sanding. But, it is easy to do damage with a scraper. Scrapers are flat, and nearly nothing on your boat is, you need to be careful not to gouge the curved areas.

The best method is to blast the bottom, but it’s not a DIY project. Professionals can blast your boat with non-silica abrasive and leave a perfectly clean bottom. Some processes are done wet, to keep the dust down to zero. This also isn’t the cheapest method, but it works very well. Beware of anyone who suggests that you sandblast your bottom. Silica abrasive can embed in your fiberglass and can cause serious issues down the road.

Once you have the bottom clean, you need to decide your next steps. If you don’t have a good barrier coat, this would be the time to do it. Even if you do have barrier coat, you probably removed a bit with the paint. A fresh coat can add protection and act as a primer for the bottom paint.

When you are ready, roll on a coat of paint, and let it dry overnight. Finish off with a second coat and you are ready to go. Be sure to verify overcoat and launch times by reading the label on the paint can.