How Much Hardener Do You Add To Your Body Filler? You need to be sure to add the right amount of hardener to your Bondo; adding more than enough will cause your body filler to dry faster and give you less time for shaping and laying your filler on your panels.

Does Bondo cure without hardener?

Without it the bondo won't get hard. Put a small piece of bondo on a board and run a line of hardener from end to end. It will take about five minutes to mix it up and apply it. If it starts tacking up, stop applying it or you'll mess it up.

How long does it take Bondo to harden?

Allow the putty to dry 15 minutes at 77 degrees F (25 degrees C). In cooler climates, putty may take 20-30 minutes to dry.