Although the resin for auto's and boat's and the like will work, there are two problems with it. First, it comes colored, usually purple, green or amber, which makes it difficult to color, it is also a lay-up resin. This type of resin is intended for thin layers and not for thicker casting situations. The fiberglass is where it gets it's strength from. There are other differences, but these are the two that will directly affect your casting.