I would fuel proof after the model is covered it will help hold the covering down arounnd the engine compartment where it always seems to pull up with age. Also covering stick best to bare wood. I always fuel proof with thinned epoxy and on airplanes that has an open engine compartment Like the Sig SE I paint the inside of the engine compartment black or some other color that matches the plane with Testors Model Master paint so you can't seen any bare colored wood.