Bottom paint can be applied directly to bare fiberglass. It should be noted that even though bottom paint can be applied to bare fiberglass and have great adhesion, it is very highly recommended to apply an epoxy barrier coat product to the fiberglass prior to applying bottom paint.
Prior to applying any bottom paint directly to bare fiberglass, the most important thing is proper surface preparation. Brand new gelcoat will have a wax that must be removed, as the wax will prevent any products from properly adhering. Dewaxer products such as Pettit Bio Blue, TotalBoat Dewaxer & Surface Prep, or Interlux Fiberglass Surface Prep, or SeaHawk S-80 Wax N Grease Killer. If the fiberglass area used an epoxy product instead of a polyester, vinyl ester, or gelcoat product, then make sure to wash the surface with a mild soap and water prior to sanding. If the area that is to be painted is epoxy based, wash the surface with a simple soap and water solution to remove any potential amine blush as well. Once dry, the surface must be sanded, commonly with 80-grit sand paper until the surface is uniformly dull, frosty appearance. Remove any sanding residue prior to applying any paints or primers, and it is recommended to wipe the recently sanded gelcoat with the dewaxer product again to ensure removal of any contaminants.
At this point, the fiberglass is prepared for the application of bottom paint or an epoxy barrier coat. It is very strongly recommended to opt for applying an epoxy barrier coat before applying bottom paint. Epoxy barrier coat products prevent blistering of the fiberglass material. Polyester an vinyl ester resins do have a sense of porosity on the microscopic level, which can allow water into the cured resin via osmosis. This can create osmotic blistering, which will look like little pox below the waterline. Though these little blisters can be ground out, filled and repaired, it is a rather laborious and tedious task to perform, and it can all be prevented by properly applying an epoxy barrier coat product prior to applying a bottom paint. The epoxy barrier coats contain mica, and when teamed up with the epoxy base, it does not allow water to pass through. A few of the most common epoxy barrier coats on the market includes TotalBoat TotalProtect or Aluminum Boat Barrier Coat, Interlux Interprotect, Pettit Protect, and SeaHawk Tuff Stuff.
Always follow the bottom paint, or barrier coat's instructions carefully for proper application. This will yield the best results, and performance of the coatings.