Are all primers the same?
Primers are paint coatings that serve as an intermediate layer between the bare surface and one or more outer coatings. Proper use of primers can spell the difference between a successful project outcome and a poor finish.
Primers are very specific in their use. They can be used either as build-up coatings or conditioners to prepare the surface for the application and adhesion of topcoats on specific types of materials.
The critical thing to understand is that primers are formulated by each paint manufacturer to work as part of a complete system. If they are not used properly, very expensive paint systems can fail. They must never be eliminated from an application system because they seem to play only a minor part or are covered up by subsequent coats.
Do you need to prime for bottom paint?
If you plan to apply the same type of bottom paint that is already on your boat’s hull, you do not need a primer. However, if you plan to apply bottom paint to a new fiberglass boat, you will need to clean the hull and then wipe it down with a solvent to remove vestiges of wax from the factory mold. Then you have two choices: either sand the hull before applying the bottom paint, or to eliminate the need for sanding, you can “chemically sand” the hull by applying a tie-coat primer that will bond to the hull and the antifouling paint you plan to apply. Note: Chemical sanding, while optional for polyester gelcoat, is recommended for vinylester gelcoat and other blister-resistant gelcoats, which should never be mechanically sanded.
If you keep your boat in the water for extended periods, you might consider applying an epoxy barrier coat, which will prevent water from seeping into the gelcoat. For more on osmotic blistering and barrier coats, see our West Advisor article on Epoxy Barrier Coats.
Do you need to use a primer?
The answer is usually “Yes.” When choosing a primer, always stick to the manufacturer ’s recommendations, which you will find on the paint can’s label. If you plan to apply an Interlux topcoat, use the appropriate Interlux primer. If you plan to apply a Pettit topcoat, use the appropriate Pettit primer—and never cross brands.