This might sound dum,but gotta ask ? A guy that ownes a body shop that rebuilds old cars and trucks. Told me he does not like bondo and uses very it little. But when he does use bondo,he never mixes it with cream hardner. Instead he mixes bondo with resin. The same resin people mix for fiberglass mat. He claims it mixes better, when dried the bondo is harder with less chance of crackin. Also its smoother to put on. So my question is has anyone ever mixed bondo with resin ? I know a little resin hardner has to be used or the resin will never get hard by its self.

 

I've got very little experience with bondo, but from what I know from working with resins for laying up carbon fiber, if you want it to set right you need to stay within the manufacturers recommended mixing ratio and use what they recommend otherwise you risk the resin not laying up correctly and either becoming powdery or overly brittle.

 

If you want a tougher, more water-resistant filler, there's "marine" body filler. It's reinforced with fiberglass. The downside is, they're not kidding when they tell you to shape it before it hardens completely. It's almost like grinding metal once it sets.

 

Bondo is a brand name and there are better ones. It's made to smooth imperfections. Check your local auto paint store for some good stuff. I usually replace anything that needs "filled" but have been using Marglass on occasion, short strand fiberglass filler, probably about the same thing. It holds up a lot better and haven't had it crack yet.