Can I use fiberglass cloth and resin to fix tiny holes in fenders? I'm trying to restore an adult tricycle and the fenders have multiple very small rust holes in them.  Can I cut fiberglass cloth and make a "sandwich" with them with the fender in the middle and then cover both sides with resin? Or would a substance like "Bondo" be better?
Since it is no load carrying part and if you keep enough clearance between the repaired spot and the tyre there should be no problem. Take care to sand away any rust beforehand and treat the rusted part with rust removing chemicals.