Finally had a chance to experiment with the Alclad Aqua Gloss and Winsor & Newton Matt Varnish.
First, the Aqua Gloss. I followed the instructions and sprayed it straight from the bottle with no thinning. I started off at around 23 PSI through my Paasche H. I was spraying at a fairly close distance, maybe 3 inches away. I began by trying to do light mist coats, but I found that it was leaving a pebbly finish. So, I lowered the PSI to about 20 and increased the flow at my airbrush tip to lay it on a bit heavier. I also started spraying a little closer, like perhaps 2 inches away. What seemed to work best is to lay it on pretty heavy, just to the point before it starts to run/pool. It looks uneven when wet, but when it dries, it levels out pretty nice. I'm going to do a few more tests, possibly at a little lower PSI, but overall I'm much happier with the result than I have been with airbrushing Future.
As for the Winsor & Newton Matt Varnish, I love this stuff. Following recommendations I had seen elsewhere, I thinned it 20% with distilled water, and sprayed it at about 20 PSI at a distance of maybe 3 inches. It covered quickly and smoothly, and dried fast to a very consistent, dead flat finish. Very easy to use, and cheap. The 75 ML bottle costs about 5 bucks and should last a while.
I want to experiment some more with the Aqua Gloss before committing it to my model, but so far I was pretty impressed with the result I was getting. It was certainly smooth enough for decal work, if not a perfect mirror gloss.