I fuel proof as I'm building using epoxy that I thin with alcohol. I get the complete compartment behind the fire wall and in the tank compartment, front of the fire wall and inside the cowl area. Then I may use Klass Kote epoxy if I have the colors needed on hand. I also use Lustrekote paint or Ultrakote paint if I need a color to pretty up inside the engine compartment. I do everything before covering but that's a choice thing.