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I'm no chemist either, nor do I like taking risks with important paintings. So I use the varnish that the National Gallery uses: Gamvar, by Gamblin. Easy, simple, looks great. (Hey, that ought to be a tv commercial!)
Damar varnish yellows dramatically over time and because it needs strong solvents when conservators try to remove it, sometimes a bit of the top layer of paint comes off with it. (You can see some before and after varnish removal pictures on some museum websites.) I try to stay away from it, even retouch varnish made with Damar.
With Gamvar, if you make a terrible mess of it (though I don't see how) you can take it all off with a more gentle solvent like odorless mineral spirits. Gamvar is glossy but it's easy to cut that glossiness by thinning the varnish with mineral sprits when you put it on, or with Gamblin Cold Wax medium in whatever proportion you like, to get a less glossy surface if that's your preference.
I always take the paintings out of the frames before varnishing, and wait til the varnish is very dry before putting the frames back on.
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