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.
A solvent (from the Latin solvÅ, I loosen, untie, I solve) is a substance that dissolves a solute (a chemically different liquid, solid or gas), resulting in a solution. A solvent is usually a liquid but can also be a solid or a gas.