I am building a GP Super Decathlon. When I was trying to smoothen the holes I opened for the engine the sanding tool touched the surface and made a small dent. Is there a way that I can fill it up before spray painting?

I have had excellent luck using "spot putty" - a Wal-Mart or auto supply store has it. This comes in a tube and is red in color.

Put in on, let dry overnight and sand with 150, then 220 paper. Then prime and paint.

If you need a "backing" to partially close the hole, use a piece of 2 oz fiberglass cloth on the inside of the cowl and adhere it with THIN CA.