Is there a reason that polyester based resin does not stick to bare metal very well? Fiber filler is based the same isn't it? It sticks fine. I mixed some resin with some 1.5 mat on a flat piece of 80 grit sanded metal. When it cured ,you could peel it right off. (with very little effort). Metal was pre-cleaned w/w&g remover.
I haven't experienced this same issue. On metal tanks with dents I use a bondo filler and cap it with fiberglass, holds like a champ. I use a short strand mix tho.