The important element for recoating the powder coating
The most important element for recoating the powder coating and as a matter of fact, for applying a different topcoating over an applied coating is to be sure the new coating will not lift or wrinkle the old coating. Check the old applied coating with a strong lacquer thinner by wetting the surface and giving it a couple of rubs with a damp cloth. If there is no excessive softening the coating should be OK to recoat with a new liquid topcoat.
Powder coating is cured and baked at high temperature classified as thermal set coatings. The powder coating is urethane so any automotive grade coating will work very well. Clean the gas tank with a strong solvent or detergent to remove all petroleum residue. Sand or abrade the surface overall with 280 or 320 grit sand paper before the application of the liquid topcoat. The existing powder coat takes the place of a primer coat.