What is the difference between water based and poly polyurethane based varnish? Is there anything wrong with using dope?

Regarding finishing, I have been looking for waterbased polyurethane varnish. It seems all the varnishes I can find are EITHER water based or polyurethane based. What is the difference. Is there anything wrong with using plain ole dope or is it too heavy?

Yes, dope is too heavy, unless you spray clear dope on in VERY light "mist" coats, and only two or so of those. The other hazard with dope is that if you get it on just a bit too heavy, the solvents in it can soak through the balsa and attack the foam underneath. With light enough sprayed coats it can give good results if you're very careful.

Petroleum solvent based varnishes can also work, but their thicker consistency makes it difficult to put them on in light enough coats, and you still have to be careful about the solvents attacking the foam.

"Water based" refers to the solvent carrier that the resin part of the paint is dissolved in. "Polyurethane based" refers to the resin portion of the paint that remains after it's dried. It's possible for what you found to be one or both of those, or neither one. What you're looking for will probably be called "polyurethane". The key is to look at the cleanup instructions. If it says it's waterproof when dry but you clean up using soap and water (not mineral spirits, turpentine, petroleum solvents, etc.), and when you shake the can it sounds and feels more like water inside, not thick and oily like regular paint, it's probably the right stuff. The other clue is when you open it, it should be thin and milky colored like soapy water, and have a very mild smell, possibly with a faint alcohol odor.

Make sure to put it on in very thin coats (fold a pad from a paper towel and get it just damp), and make sure to let each coat dry thoroughly before sanding lightly to remove the "fuzz" and applying the next coat. Coat both sides of the panel at the same time to avoid warping. Remember also that the goal is to protect the wood from moisture, NOT to get a completely filled "glass smooth" finish.

Don Stackhouse @ DJ Aerotech