Can I Use Thinned Bondo to Pour Into a Mold?

Undercuts

• Bondo-resin is very hard when it sets up. As a result, you have to avoid using it in rigid molds that contain undercuts. An undercut is an area on the mold that allows the Bondo-resin to seep in, locking the two together upon hardening. Fill undercuts in rigid molds, such as those made of plaster or fiberglass, with modeling clay when using with Bondo.

  Undercuts are not a problem if you are pouring thinned Bondo into a mold made of silicone, latex or another flexible material. Even if an undercut is present, the mold can be peeled off of the hard Bondo-resin.

Mixing and Catalyzing

• Mix Bondo and fiberglass resin in a disposable paper bucket. Add the Bondo first, then thin it with resin until it flows like maple syrup. Use a paint stick to combine the two materials. Once the mixture is ready, add the catalyst for both components in the correct proportions. It is very important to mix in the proper ratio of catalyst, which is provided on the Bondo and resin containers. If the mixture is over-catalyzed, it will harden faster than it can be used. If it is under-catalyzed, it may never harden at all.

Pouring or Brushing

• Pouring a small, solid object with Bondo-resin is relatively simple. Pour the mixture slowly into a mold and allow it to harden. If you are making a larger object, however, you may want to brush in a layer of Bondo-resin to be backed up with another material. Brush in the Bondo with a cheap paint brush. The layer of Bondo-resin should be 1/4 inch thick.

Backing it Up

• Once you have a layer of Bondo-resin brushed in and hardened, you must back it up with a stronger material such as fiberglass, which naturally bonds to the first layer. Any material can be used, however, from plaster to self-expanding polyurethane foam.