When applying short strand fiber glass to fill in a small hole, dose it have to bond directly to metal or is a sanded paint ok to apply it too

Given a choice, I'd grind off the paint and allow the grinder to leave jagged scratches through to the metal. The bond to the paint will only be as good as the paint bond to the primer below and it's subsequent bond to the metal.

In either case, I was taught to GRIND the surface which is to receive the patch. That way it has both a MECHANICAL bond as well as the CHEMICAL bond of the resin.

FWIW

E

I have found that fiberglass will bond to metal for only a period of time before seperating. The amount of time will be based on how much body flex there is at the repair point. If the repair is going to be seen, ie. a body panel. You would be best served to relieve the area with a small ball peen hammer. Use an amount of body filler to skin that area and then glass over the hole. Body filler is made to bond to metal and flex more than fiberglass. Fiberglass will stick to the filler better than it will to metal. That is the method I used 12 years ago to "glass" a set of quarter panels to my Z. They are still there and no cracks. Good luck.