I've just scrapped 20 years of bottom paint off my hull with a traditional double-edged paint scraper. Now that the gelcoat is exposed, how should I clean the hull and finish or reseal the bottom? Should I sandblast the paint off, or is it too destructive?
Don Casey responds:
Assuming you don't have blisters, which you didn't mention, then the next step is to wipe the exposed gelcoat with a wax removerInterlux 202, Awl-Prep, or similar, using plenty of clean rags. Unless the hull was wiped down before the first coat of paint was appliedvery doubtfulthe gelcoat still has mold release residue which you will drag into the microscopic scratches you are going to make when you sand, and this will forever interfere with the bond of the paint.
Next, load your palm sander or random orbit sander with 100 or 120 grit paper and lightly sand the entire bottom to remove residual "islands" of old paint and to give the bottom uniform tooth. Wash the bottom thoroughly and wipe it with the solvent recommended for the bottom paint you have chosen. You are ready to paint.
A barrier coat on a healthy bottom is an unnecessary precaution unless you 're planning to take the boat to the tropics and use it there for several years. Even then, I would be reluctant to incur the expense, add the weight, and risk the consequences of adding additional chemicals into the mix until I saw real signs of osmosis.
By the way, good move in not letting the yard sandblast. Odds are shamefully short that your gelcoat would have suffered severe damage. Don't give up that benefit by sanding too heavily.