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.
Oil-based polyurethane is simply a type of varnish. Itâs common alkyd varnish made with some polyurethane resin added. Alkyd is the resin used in almost all varnishes and oil-based paints. The polyurethane resin adds scratch, heat, solvent and water resistance to the alkyd varnish. Pure polyurethanes (with no alkyd resin) are always two-part products.