While some types of paint are formulated to resist scratching, others are not. If you've applied a low-sheen latex paint to a surface subject to duress, abrasion scars and stains may ultimately result. Fortunately you can prevent scratching by applying a durable, clear, protective coating that will prolong the life of the finish and keep it looking smooth and free from subtle flaws. To prevent cracking and shedding, choose the appropriate type of clear sealer based on the type of paint you're working with.
Wash interior painted finishes with a water-based soap to prevent poor bonding of the clear, protective finish. Rinse the painted finish with wet rags. Lightly rinse exterior painted finishes using a pressure washer. Let the paint dry for at least four hours.
Lightly sand the painted finish with 300-grit sandpaper.
Place canvas dropcloths beneath the painted finish and frame the surface by applying painter's masking tape over adjacent surfaces.
Coat the painted finish with the appropriate polyurethane, using the proper application tool. Apply water-based poly to latex and acrylic painted finishes, using a polyester paintbrush. Apply solvent-based poly to oil-based painted finishes, using a natural-bristle paintbrush. Add a very light coat, applying gentle pressure with each stroke. Let the poly dry for four hours.
Lightly sand the poly finish with 300-grit sandpaper; then add a second coat.
Wash polyester brushes three times using water; wash natural brushes three times using mineral spirits.
Certain paints are so durable they don't require a clear polyurethane finish. If you need to paint a surface that will have to endure a lot of friction, use an acrylic enamel or epoxy paint. To prevent fading by ultraviolet light, don't use epoxy paint on exterior surfaces.