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.
What is the difference between polyurethanes and urethanes? Polyurethane coatings were invented in the mid-20th century. Plastic resins in liquid form, they are used primarily to protect wood surfaces. Their main ingredients include acids derived from vegetable oil, nitrogen-based chemical amalgams, isocyanates, and mineral spirits.