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.
solvent - a liquid substance capable of dissolving other substances; the solvent does not change its state in forming a solution dissolvent, dissolver, dissolving agent, resolvent acetone, dimethyl ketone, propanone - the simplest ketone; a highly inflammable liquid widely used as an organic solvent and as material for making plastics