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 based epoxy systems are not sold through home hardware stores, which is the form of epoxy that is the most durable and offers the longest life. Only water based epoxy systems are sold through Home Depot and Lowe's, which are great for residential applications, but do not offer the longevity that the solvent based epoxy systems do.