Bondo is the generic term for auto body filler. It will be used in many areas of trailer restoration. It will cost you about a gallon and it comes with a tube of hardener. It can be purchased at auto body supply stores and even at walmart type stores if you're desparate.

To mix, follow the instructions on the can but in general, you'll put some on a non porous surface, add a small amount of hardener and mix with your putty knife.