I recently used Army Painter spray varnish on some resin terrain pieces and it seemed to work very well. I did a coat of gloss underneath and then finished with a coat of the A.P. anti-shine.
However, given what I've read here and on other threads about spray varnishing in general, I believe that when I begin completing painted units in the next few months I will utilize brush-on varnish.
The one spray varnish that I DO trust is Testors Gloss and Dull Cote. Those two seem to be a wargamer's choice for good results.
I will experiment with both varnish and brush-on as I'm varnishing units and carefully move forward from there.
Ironically, I've heard that the Army Painter matte brush-on varnish is really excellent and you avoid any potential frosting or graying that you might get with the spray.
But then again, as was said above, with ANY spray product there are going to be situations and conditions where some kind of problem occurs. The dreaded "frosting" that so many have cursed I have seen happen with the most expensive modeling varnishes as well as the cheaper hardware store brands.
weighs the pros and cons of using a power drill on the minis workbench.