1. Some of the wood varnishes, even though labeled as clear, still have a slightly yellow tint to them, or will yellow over time. Approach with caution.
2. If it's an enamel, solvents may or may not be a problem depending on the ingredients (There are acrylic enamels, polyurethane enamels and others); HOWEVER, you'll need to experiment on some old stuff if you're dead set on trying it.
3. Because it's for wood, what comes out of the can may be considerably thicker than what you would normally spray on plastic. There may also be a considerable difference in drying times. Again, experiment.
4. Remember that you usually get what you pay for. If you get really good results, let us know! Good luck...
Gip Winecoff