Just seen this thread and thought I'd add my 2 p worth.
I bought some Vallejo gloss varnish and their thinners a while back to test it, because I build slot cars for people and they wanted their decals protected.
Only today was the weather suitable. So out with my nice new Paasche Model H, which I recently bought specifically for clear. I have used a Model H for years and recently had occasion to buy spares for it so bought the new one at the same time, for very little money direct from their Seattle base via an American chum. Hooked it up to the rewired old Sprayit compressor and off we went at about 20psi and 6-8" distance. I thinned the varnish about 65% varnish/35% thinners, swirled with a brush to mix it, unscrewed the nozzle till a nice 8mm diameter fan pattern was there and did a Lotus 24 as if it was a real car. Like a robot, is my advice, with a little overlap to keep the wet edge going all over it.
When I left it an hour ago it was glinting in the sun like no old Lotus ever did!
Decals unharmed too. Not too thick though as the stuff goes a bit milky, though does end up clearing and is a very smooth coat indeed.
Valejo and Paasche Model H very highly recommended. I think one problem with most on these sites is that they will try to spray finish coats with what is really a very fussy graphics instrument for which I have never found a use....including graphics, in which field I once worked.
Cheers,
Martin