When repainting a Riptide motor the best method would be as follows:
Remove all loose paint by scraping it off. Sand area with a medium grit sandpaper and blend in areas where the paint and substrates meet.
Clean area to be painted (VERY IMPORTANT): A strong cleaner (paint thinner) that removes dirt and oils and does not leave any residue will work best. Before painting wiping the area with isopropyl alcohol will help remove any remaining residues and moisture.
Primer Coating: Use an epoxy based primer. This will give the strongest adhesion and corrosion resistance. Allow primary to dry completely per the manufacturer instructions.
Final Coating: Use any Acrylic based top coat. Acrylic spray paints offer UV resistance as well as durability. Again the surface should be clean, dry, and free of any residues. Suggest paint is Cardinal A-4101-67194 (Aerosol, touch up air dry acrylic enamel).
Note: For each of the individual products mentioned above it is always important to follow all the manufactures directions and safety precautions.