I use the Sculpey gloss & satin glazes and also have been using Varathane recently. I tried nail polish but wasn't happy with the results. Maybe a specific brand works better than others?
Never use nail polish. Not only can it wear down much more quickly than a proper glaze but it can yellow or flake over time, ruining your product. I use Sculpey gloss and satin glazes.
I mainly use the Sculpey glaze, but if I want something a little thicker for a syrup or something, I will use Glossy Accents by Ranger. However, I have found that this gets really sticky and nasty :-/ but putting a thin coat of the sculpey glaze thankfully solves that problem.