If your boat has been painted too many times you will find that the paint is building up to the point of no return. With too many coats the layer closest to the hull will eventually lose its bond due to the age of the paint and the sheer weight of the paint covering it. We have tried chasing these areas as they pop up and ended up with patchwork bottoms. There is an alternative however.We usually recommend soda blasting to remove the old paint. We understand the environmental challenges associated with this and hire this task out to people who are totally compliant with all the current regulations. They tent up the boat to contain the mess and haul it away when it is done so we can do what we do.
After the old paint is removed we generally have some hand work to do. You don’t want the blaster to get too aggressive and get into the gelcoat. There will be spots where this does happen. I’ve never been in the tent when the blasting is going on but I would imagine visibility is not all that great. We look for spots where the gel was compromised and use an appropriate filler to take care of those spots. It is also time for a bit of fairing to ensure the smoothest bottom possible. I have seen some do-it-yourselfers skip this step, but if you take it that far why not get the best job possible?
With the bottom paint all gone you should use some primer before the first coat of paint. Pettit and Interlux both make bottom “sealers” that are used for barrier coats. You can just use a single coat and they will act as a primer but if you wish go ahead and put on a few extra coats for the barrier coat protection – it’s never a bad idea to have a bit of extra protection. Just be sure to follow the instructions or you can end up with an adhesion problem like we spotted this year on a boat that was in for storage.
When it is time to put on the new bottom paint, all manufacturers recommend 2 coats. Our technique is to use a hard paint for the first coat. That way you never wear through to the undercoat. You don’t have to worry about using a high quality paint for this coat as it should never see the light of day. We don’t use cheap paint but by no means use expensive paint. You should also use a different color on the first coat for two reasons. First using a different color makes it easier to not miss any spots when you put the second coat. Also you should use an ablative paint for the second coat so you can see when you have “ablated” through to the first coat.
I do want to reiterate that you should be using ablative paint for subsequent coats to avoid future build ups. And don’t over apply it. A thin coat works and most of the ablatives these days are multi-season paints. Since you don’t need to paint every year, you can afford to use a high quality paint. If you only paint when necessary you should never have the build up problem again and can spend more of your time cruising – and will probably gain a bit of speed in the bargain.