Thanks to Don, Clay, & Henk for responses about Liquitex varnish -- very helpful. Not that I might not end up with something completely different but it's reassuring to know that others have done it in various ways and the world kept spinning on its axis.
For what it's worth, at this point I'm thinking in terms of very small prints with a very high gloss -- a sort of reference to glass. (Tho I haven't tried that yet, it may be too awful to even consider... tho then again "awful" may be just the ticket.)
But a question for Henk: You said for inkjet you spray lightly before applying varnish... I take it that's because the ink might run so you more or less "set" it first? My particular inkjet ink doesn't seem to run if it's on paper (as opposed to plastic) & allowed to dry, but pre-fix sounds like a good idea anyway.
It's probably no big deal to throw out some "stinky" varnish that's growing things, but I'm trying to remember what I used to put a drop or two of in the top of a jar of liquitex paint that was growing mold. It may have been a much diluted formaldehyde. I'll look around the studio & see if inspiration strikes--- sometimes things come back when you go on auto-pilot.
(As when I forgot my ATM PIN number -- not as braindead as it seems, because I hadn't used it in years, since I was banking elsewhere, etc. etc. etc. But finally I had to go to the bank IN PERSON and have the old one -- which they evidently can't or won't access -- killed & think up a new one. And then I went to the machine for the transaction -- and incredibly -- or not so incredibly -- while I was going through the motions, suddenly a voice in my head sang along with the OLD PIN number.... )
But as I was saying, so far I'm thinking formaldehyde... If you can get it of course. Maybe the same friendly undertaker who supplies the cremains?
Meanwhile, thanks again...