The Blue Bottle Tree - This isn't resin. This is sanded and buffed. There is no sealer, coating, varnish, or glaze. Just polymer clay. And no, it doesn't have to be hard.
How to get a matte resin finish on polymer clay charms! - YouTube
Understanding Polymer Clay Glazes, Sealers, and Varnishes
Glazing Polymer Clay With Resin! | Crafty Amino
How To : Glaze Your Polymer Clay Charms With Resin - YouTube
Amazon.com: Sculpey® Gloss Glaze, Non Toxic, 1 fl oz. bottle with precise flow twist cap. Will add a glossy finish to your baked polymer oven-bake clay creations!
What to Consider When Using UV Resin on Polymer Clay Charms – Sculpey
Amazing Jewelry - Polymer Clay & Brilliant Resin - bake embed or glaze – Little Windows Brilliant Resin and Supplies