Clean all the soap scum and buildup that may be in the tub using a mild detergent and a scrub brush. Rinse the tub with clean water and dry it out with a towel.
Put on a respirator and evenly sand the entire tub using a palm sander and 100-grit sandpaper. When the tub is completely scuffed with 100-grit, switch to 300-grit and resand the surface.
Wipe the tub out with a clean rag and acetone. Remove any dust, grease or any other residue that may be in your tub.
Tape off any metal fixtures and the perimeter of the tub using painter's tape.
Mix the gel coat and catalyst together in a small bucket using a stir stick. Follow the container's recommendations for the correct amount of catalyst. Add the gel coat to the spray gun and apply the first coat in a heavy mist. Hold the spray gun at least 18 inches from the tub at all times and apply the paint as evenly as possible. Let the first coat harden.
Apply the second and final coat to the tub so it solidly covers the entire surface. Spray this coat fairly heavily but avoid any runs in the paint. Let the gel coat harden for 24 hours before running the water.
Remove the painter's tape from the tub.