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.
Spar varnish is an older formulation of varnish vs. Urethane Varnish is a modern formulation of a tough clear coating used primarily on wood surfaces exposed to outdoor conditions.rethene, naturally is a urethene based product, whereas varnish in the traditional formulas had linseed oil as well as various hardeners. On the old formulation the varnish never completely dried. This was so it would not dry hard and crack when exposed to great heat, UV light and salt water.