I've found that, the more hardener you add, the faster it sets up, but the more brittle it is. It also doesn't bond as well with the thing I apply it to if i add too much hardener. So I rely on the runny super glue to insure a good bond.
Before I top coated it with crazy glue, I had filler over rattle chambers pop off. Once I started top coating with the runny glue, no more pop offs. So I do it to everything I bondo now, just to give it that added strength and hardness.