I agree that bondo isn't the best. I always use epoxy with filler. Still, bondo does work and holds up well if you paint the finished repair--which I always do. You must paint over epoxy too in order to keep the resin from degrading in sunlight. So it's a moot point. Gel coat material of any kind really doesn't add any strengh, per se, to the boat so it's a moot point. It's just cosmetic.