A simple wash with clear water
removes amine blush. No soap, no solvents. Then sand that washed surface with 80 grit paper, to provide the gelcoat
with sufficient key so it won't run. Be sure to use non-air inhibited gelcoat
that has a paraffin wax added.
Non air inhibeted gelcoat does not have wax added and is what is generally used for repair finishing. As the final step after spraying the gel on the repair the area is sprayed with PVA to seal the repair from air and aid the cure.
Professionals frown on the use of wax added geclcoat for exterior finish work. If you are really concerned about adhesion of poly based gel to epoxy
repairs there are companies that make primers "Duratec" that is claimed to act as a tie coat.