Boat Paint and Marine Painting

Preparing Your Boat for Bottom Paint

If your boat's bottom paint has deteriorated as badly as this one, it will likely need to be sanded thoroughly with a drywall sander, then pressure-washed, before it can be repainted.

Preparing your boat properly for a bottom paint job is important.

1. Clean the bottom thoroughly.
   If you use a bottom cleaning service, have them clean the bottom the day before your boat will be hauled for the bottom paint. If you prefer to clean the bottom yourself, you can arrange with the marina to have the haulout done 2 or 3 days early, which will make it easier for you to clean the bottom well without having to don scuba gear and do the cleaning while the boat is in the water. Many marinas and boatyards have a high-pressure spray that will make the cleaning easier.
   
   Scrub the hull with a coarse bottom-scrubbing pad, rinsing frequently with fresh water. Then, using a block sander with 80-grit wet-dry sandpaper, sand the top edge of the waterline to about 6 inches down, and sand smooth any chips in the existing paint. Avoid sanding into the boat's gelcoat, and keep the sandpaper wet through frequent dippings into a bucket of fresh water.
   
   Start cleaning as soon as possible after the boat is hauled out. If barnacles, weeds, and slime are permitted to dry, they will harden and will be much more difficult to clean off.
2. Inspect the hull
   Check for cracks, separation, blisters, or other problems. In particular, do a careful inspection where the keel and through hulls join. Use sandpaper to open any blisters, and thoroughly clean out any separations or cracks, then use marine epoxy to make any needed repairs.
3. Make any needed repairs
   Check for cracks, separation, blisters, or other problems. In particular, do a careful inspection where the keel and through hulls join. Use sandpaper to open any blisters, and thoroughly clean out any separations or cracks, then use marine epoxy to make any needed repairs.

Repairing Cracks and Separations

The most common method to open blisters and clean out cracks and separations is by grinding or sanding. It is important to wear eye protection for this task, because blister fluid is often under surprisingly high pressure — as much as 200 PSI. You'll need an air or electric polisher with 1000-2000 RPM, with an 8-inch foam sanding pad, for this job.

Place a tarp underneath the boat to catch debris, and dispose of properly.

Starting with coarse grit, grind the hull to strip the bottom paint, open any blisters, and clean out cracks and separations. Avoid gouging the hull by holding the grinder at a low angle of 5-10 degrees. Remove enough of the gel coat around separations or cracks to expose all of the damaged area. Continue grinding until a solid, undamaged surface is exposed. For blisters, remove gel coat until the bulge of the blister is flush with the bottom, then grind the blister surface again with a finer grit to remove the coarser scratches and fair the surface. If sanded fair enough, little filling and fairing will be needed for the blisters after sanding. When you're done with all the sanding and grinding, flush the surface thoroughly with fresh water while scrubbing with a coarse nylon bottom scrubbing pad or a stiff brush.

Let the boat dry overnight, and make sure the hull is completely dry before starting to fill any cracks. If the hull is still wet or damp the next morning, you can use fans or heaters directed at the surface to help speed up the drying, so that you can start your epoxy repairs sooner.

Wetting Out

Use a disposable paintbrush to paint the areas needing repair with an unthickened epoxy mixture (5 to 1 ratio hardener.) You can usually buy a pump with the epoxy that will provide the correct mixture ratio. The epoxy mix will saturate reinforcing fibers that are exposed, making them stronger, and will provide a good base for the thick fairing mixture. Be sure to saturate every cavity with the epoxy mixture, and work the epoxy into cracks. Let this dry for 30-60 minutes before applying the thickened epoxy mixture.

Your boat's hull will be beautiful once again after a fresh coat of bottom paint.

Applying The Fairing Compound

Make your fairing compound by mixing resin and hardener in a 5 to 1 ratio, and add filler to a non-sagging, peanut butter consistency. Apply the fairing compound into the prepared cavities and blisters with a plastic squeegee before the unthickened epoxy reaches its cure stage. (Read the directions for cure time for the specific epoxy you used.) Use the squeegee to smooth the epoxy mixture so that it's slightly higher than the surrounding surface, and remove any excess epoxy before it cures. Allow the epoxy fairing compound to cure thoroughly before sanding.

Start Sanding

Now you're ready to sand the fairing material so that it forms a smooth surface with the surrounding contours. Use a power sander to remove most of the epoxy filler on the repaired areas, then use a 50-grit sandpaper if you have a lot of fairing material to remove. When approaching the final contours, use 80-grit paper with a block sander or hand sand when close to the final finish. Remove the sanding dust, and inspect the surface again. If you have the time, you can mix a new batch of epoxy filler and repeat the above steps, or you can fill any remaining voids with a polyester filler such as Bondo, which usually hardens in minutes. Sand again with 80-grit paper.

Final Steps

After letting the epoxy cure overnight, wash the hull again, rinse it thoroughly, and make sure it's completely dry. The hull is now ready for its bottom paint.