Laminating epoxy should be used to laminate fibderglass after the pinitrating epoxy has completly cured A light sandinf on the serface with 50-60 grit sandpaper helps adsion. A good filler can be made with very fine sawdust(I save the sanding dust from sanding to make filler.If strength is not a factor I have used corn sterct to make a filler. In the old days we used to have bags of powdered asbostas to make filler we didn't know the dangers. Fiber glass coatings sell different types of fillers check their catalog. If you lived closer i would be glad to lend a hand.
Jim Smith