Historically it has been found that Polytone adheres very poorly to hard surfaces like fiberglass. Typically if used there an epoxy primer was sprayed first and after solvent evaporation, but before curing, the Polytone was sprayed on. A friend after doing that on his build many years ago was recently attempting to remove the Polytone top coat with solvents and no go. An automotive urethane works well on the fiberglass parts.