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 based glues are made of natural or synthetic polymers, mixed in water, and are used when one of more surface is porous (e.g. wallpaper paste). They are slow to dry and will not stick non-porous materials like plastic. Solvent based glue are a synthetic adhesive polymer mixed in solvent.