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.
In the case of opaque plastic and urethanes left exposed and dirty, where the surface is eroded and oxidized, dried out and faded, you can revitalize it with a plastic restorative agent. MothersÂ® Back-to-BlackÂ® is a good example of a formula designed for this purpose.