Rapid Prototyping involved creating a working model of various parts of the system at a very early stage, after a relatively short investigation. The method used in building it is usually quite informal, the most important factor being the speed with which the model is provided.
There are water-and solvent-based -- also called oil-based -- polyurethane varnishes. Water-based polyurethanes are more environmentally friendly and safer to use than solvent-based, but they are not as resistant to wear and tear as solvent-based products.
Polyurethane (a.k.a. 'Urethane') Polyurethane (also known as simply urethane) is a popular material for body kit parts. Because polyurethane is more difficult and costly to produce, parts made from it are more expensive to purchase. However, the as-delivered fit and finish level is usually better than fiberglass.