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.
Water is often called the universal solvent. It dissolves polar compounds through dipole-dipole interactions. Compounds with oxygen and nitrogen groups are stabilized, and solvated, through hydrogen-bonding interactions. This is particularly important for alcohols, amines, and amides.