Glass-reinforced plastic is a technical name for what is commonly known as fibreglass. Boat hulls constructed with GRP are finished with a gel coat, essentially a polyester resin sprayed into the boat's mould before the fibreglass is laid down. These boats can be painted in similar fashion as cars, except you have to use a marine paint rather than automotive paint. And unlike cars, boats have a lot more surface area, so you may have to purchase twice as much paint as you would use for an automotive painting project.
Clean the glass-reinforced plastic boat thoroughly with soap and water. Doing this removes surface contaminants, such as dirt and mud, that may damage the paint.
Wipe the surfaces you are painting with wax and grease remover. This chemical helps remove smaller surface contaminants that prevent paint from sticking to the surface when it is sprayed.
Scuff the gel coat surfaces of the GRP boat you are going to paint using the grey automotive scuff pad. No shiny surface can be present. Scuffing gives the paint a surface it can properly adhere to, while shiny and smooth surfaces don't allow the paint to grab onto anything.
Mask the areas of the GRP boat you do not want to paint using masking tape and paper. If you are painting the GRP boat's gunwales, be sure to mask the interior of the boat completely, as painting from the angles necessary to get to these inner rails will almost certainly cause you to overspray into the boat's interior surfaces. Other parts that should be masked include the engine, trim and windows.
Spray the GRP boat with the marine paint using an HVLP spray gun fed with an air compressor. Marine paint comes in two types. The base/clear style of paint is more expensive, but it is more resistant to damage and more easily repaired if scratched. Single-stage paint is less expensive and easier to use than base/clear coatings, but provides less scratch resistance and ease of repair.
Sand the painted surfaces with the 1500-grit sandpaper and water after they have been allowed to dry for at least 12 hours. This step may not be necessary, however. After the paint has been sprayed, look carefully at the finish. If you can see a clear reflection from 6 inches away and you are pleased with the texture and feel of the finish, this step can be bypassed. If you choose to sand, buff the paint with an orbital buffer and the same type of polishing compound you would use on your car. This will provide you with a perfectly smooth finish.
When painting, keep the HVLP spray gun approximately 10 inches from the surface and work in slow, sweeping passes, overlapping each previous pass by 1/2 inch to get the proper paint depth you'll need.
Wear safety equipment when painting in an enclosed environment. Many marine paints are toxic until dry.