I finally remembered to look in the book Creative Clay Jewelry; it says
that Both Sculpey and FIMO have their own varnishes in both gloss and
matte, but they are applied with a brush. It suggests experimenting
with different sprays, as not all of them dry well on polymer. Two
recommended sprays are Duncan Super Matte Ceramic Sealer (matte) and Rub
'n' Buff's Spray 'n' Seal (gloss).