: Just out of curiousity, is there a difference if you are using solvent based
: or water based polyurethane? Can you add one layer over another?

I don't see why not. Water based varnish still contains resins, oils and thinners, they're just in tiny globules suspended in water. As the water evaporates, the globules coalesce and form a varnish layer. Once it's cured, it shouldn't be any different than any other varnish layer. However, I haven't done this myself, so perhaps someone else can provide impircal data on the subject.

BTW, I alsways suggest to people to pick up a copy of Bob Flexner's book, "Understanding Wood Finishing". It has clear, concise explanations of how common finishing products REALLY work and it debunks a lot of popular myths. I consider it a must-read for anyone interested in woodworking.