How do you cut it out?
People with smaller helmets generally trim their visor with a Dremel tool. You can see some dremel action in the double visor tut in the molded armor section. Always sketch out how much to cut beforehand since visors aren't cheap
Can I put fiberglass on the armor, even though i already spray painted it?
You won't want to fiberglass the
exterior of the armor, with or without paint on it. Fiberglassing should be done to the interior so you don't lose the angles and details. If you need to reinforce your armor you can attempt to carefully 'glass the inside. The majority of your paint should be ok, but resin and cloth are messy things so expect to do some touch up work.
Ideally, the steps in order are to: resin the outside (1 or more), fiberglass the inside (1 or more), sand/detail/glaze/etc., and then paint it. I'm guessing you just got anxious to see what it looked like
Hope that helps!
-Vex