Hi!
Corvin...fasten glassfiber to any surface is not done with 5 or 30min epoxy! Instead we use epoxy resin, that is 24 hour epoxy which is more like thick water [] in consistency which dries glass hard and is easy to sand. 5 and 30min epoxy is more "rubbery" in consistency, not what you should use together with fiberglass.

But back to the covering issue using glassfiber to cover holes in a trainer fuselage.
This should not be attempted because covering lightning holes in fuselage is much more easily done with either silk and dope or a using plastic covering films like Oracover or Oratex, Solartex and the like.
If you still want to cover the front part on the airplane with glassfiber and resin (either 24 hour epoxy or polyester resin) ...go ahead ...but stop at the rear and cover the holes with silk and dope or the more modern fabrics or films ...and then paint the airplane. Don't try to cover the holes with glassfiber ...its not the way to do it! It will only render your airplane heavier then necessary.

Regards!
Jan K
Sweden