For best results, painting any surface requires proper surface preparation, priming, and attention to personal safety. More so with sanding and surface prep for antifouling paints because of the toxic biocides they contain.
But just how much bottom prep do you really need? It'll depend on hull condition and the state of the previous antifouling coating.
Start by cleaning the surface so you can see what you're up against.
After haul out, when the boat is still wet is the easiest time to remove weeds, slime, and hard growth by pressure washing. Wait till the gunk dries and the job will be much more difficult.
Removing old antifouling paint
Stripping previous antifouling paint is an undertaking. Reasons for removing old antifoulants include:
- The need to repair gelcoat blisters before applying an epoxy primer and barrier coat then overcoating with new antifouling paint.
- If you're applying a new antifouling coating that is incompatible with the previous antifouling paint.
- Antifouling paints have a limited lifespan, which can be shortened by a number of factors. Evidence that it's time to remove all the old stuff include antifouling paint that shows cracks, crazing, peeling, chipping, or flaking. Even the best epoxy primer and barrier coat can't stick to this type of damage, which means the antifouling applied over the barrier coat won't stick, either.
If any of the previous antifouling is lifting or flaking off, use a small, sharp instrument to test the edges of the lifting or flaking sections. If the paint peels off easily, you'll need to strip it.
The most common DIY methods for stripping antifouling paint include chemical strippers and soda blasting. Soda blasting is a milder form of abrasive blasting than sandblasting, and involves applying a high-pressure stream of baking soda (sodium bicarbonate, but with a much larger particle size than that found in the household version) to a painted surface.
Chemical strippers are very effective at removing antifouling paint. Just be sure the stripper you use is effective on antifouling paint, and is formulated especially for use on fiberglass, otherwise it can ruin the gelcoat. TotalBoat TotalStrip will do the job well, and it's odor-free, biodegradable, and doesn't require acid wash neutralization. Simply use water for cleanup and let the surface dry before proceeding to solvent wiping, then sanding.
If you decide to sand to remove layers of antifouling paint, be sure to solvent-wipe the surface before sanding to remove any wax, grease, dirt, or oil contamination, and allow solvent to flash. And be sure you're wearing proper protective gear: a proper, tight-fitting respirator, Tyvek suit with hood, eye protection, and gloves. Finally, your sander should be equipped with a vacuum hose to catch the toxic sanding residue.
Dewaxing and Dirt and Grease Removal
Primers and paints adhere best to surfaces that are prepared properly. It's important to remove all traces of contaminants before sanding. If not, sanding will grind any contaminants into the surface and will cause improper adhesion of the antifouling paint, or primer and paint.
On a bare fiberglass bottom that is previously unpainted, it is especially important to use a dewaxing solvent to remove the mold release wax.
For best results, use the dewaxing solvent or other material recommended by the paint manufacturer.
Apply dewaxing solvent with the two-rag, wipe on/wipe off method, using a clean rag to apply the solvent, and a separate rag to wipe it off. Change rags frequently to prevent the spread of contamination.
If the previously painted surface is in good shape, light sanding will suffice. Be sure to solvent-wipe the surface before sanding to remove any wax, grease, dirt, or oil contamination, and allow solvent to flash.
For your safety, before sanding, put on your protective gear: a proper, tight-fitting respirator, Tyvek suit with hood, eye protection, and gloves. Ear plugs are also a good idea, since this job takes time and is very noisy.
To ensure optimal adhesion, sand the prepared surface lightly with 80-grit sandpaper in a random-orbit sander that has a vacuum hose connection attached to contain the toxic dust.
Use masking tape to tape the waterline before applying antifouling. It's best to use a solvent and water-resistant tape, especially for lengthier bottom painting jobs that require two or more coats. A clean-release, or safe-release, masking tape lifts off easily, leaving no gummy residue.