How to Paint Fiberglass Hull Interiorsby Gus Stephens
The interior hull of a fiberglass (GRP) boat is usually a bare fiberglass or previously painted surface. Hull interiors are not commonly gel coated like exterior fiberglass surfaces. Applying and maintaining a proper coat of paint to the hull interior adds cosmetic value and also helps prevent mildew and resultant odors in moist areas in proximity to the bilge. Staining and scratching of the fiberglass surface is also reduced by an intact paint barrier. Marine paint formulated for hull interiors is known as "topside" paint, suitable for application above the waterline. Topside coatings are approved for any interior hull application except inside the bilge. For durability and adhesion to a bare fiberglass hull interior, a one- or two-part topside polyurethane acrylic is recommended on top of a marine primer.
Sand the hull with 220-grit to 320-grit sandpaper after making sure the surface is clean and dry. Wipe away all sanding residue and sand the hull again with 80-grit sandpaper. Wipe away the sanding residue.
Apply a single coat of a one-part conventional yacht primer and allow to dry.
Apply two or three coats of a one-part marine topside polyurethane acrylic paint. Use the roll-and-tip method of application for a smooth, sprayed-on appearance. Work with small areas, only 2 square feet at a time. Apply the paint with a roller first for maximum coverage. Before the paint begins to dry, immediately follow up with a 3-inch brush lightly loaded with paint to brush out the "stippled" appearance caused by the roller and to remove the roller lines. Observe the manufacturer's recommended drying times between coats.
Sand with 220- to 320-grit sandpaper after each coat is dry, . Remove the sanding residue by wiping down the surface with a cloth wetted with mineral spirits or commercially available brushing liquid formulated for use on fiberglass. Do not sand the finish coat.
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