One of the hardest tasks as a beginner is to decide what clay to use but once that is set then you have to apply it to some kind of armature. Now that may sound easy but just imagine a full size vehicle, let’s say Sequoia size, the frame work had better be up to the task or you’ll have a disaster on your hands. Just imagine, up to 10,000lbs in weight of steel, wood, foam and clay.
Having said that, if you are making a scale model then it’s not as critical, you can get away with a wood base board with styrofoam glued to it or even a total styrofoam block. The main point is to reduce the amount of clay that you are going to use without compromising the design and stability of the form. If the clay is too thin then there could be a problem with clay strinkage and cracking due to the movement of the foam, if it is too thick then there becomes a weight issue, plus the more clay used the costlier the project. The idea is to shape the foam to ensure an even thickness of clay over the form. An ideal thickness for a scale model would be 30mm or 1 1/4 inches, this allows for plenty of artistic development.