You can mix the hardener on pretty much anything clean. Just remember that it'll be more or less ruined afterward. This batch was mixed on a cardboard sandpaper package. A nifty reusable filler batcher is a plastic frisbee. The filler won't stick to plastic, so when you're done you can pop the old filler right out cleanly.
Follow the directions on the filler can to mix the proper amount of hardener with filler. Mix the two using a rigid plastic spreader. Don't forget you have a limited working time once you mix it!
