A raw fibreglass surface often rejects latex paint. Common latex paints use an acrylic resin for bonding, and this resin does not adhere well to slick surfaces. If you're using an acrylic paint on fibreglass, it will yield patchy results and a lot of dripping as the paint fails to stick. If you want to use an acrylic paint on fibreglass, you need to sand the surface and coat it with the right primer. With this prep work, the acrylic latex should adhere without any problems.
Sand the fibreglass surface with 150-grit sandpaper to create a rough surface for paint better adhesion. Even if your fibreglass is already textured, use sandpaper. Textured fibreglass is still very nonporous, which will lead to poor adhesion, and sanding helps with this problem.
Wipe the sandpaper dust off the fibreglass with a damp rag.
Apply bonding primer to the acrylic surface with a brush or roller. Bonding primers are formulated for better adhesion to nonporous surfaces like fibreglass, ceramic and laminate. Wait for the bonding primer to cure. It will appear dry long before the primer has fully set; the primer's container will give you the most accurate drying times, since each manufacturer's formula dries at a different rate.
Paint the surface with acrylic latex paint. A roller will save you time when painting a large surface. Smaller items and tight spaces are more easily painted with a brush.