What do I use to seal or fix decoupage artwork?
In addition to clear epoxy resin, you have other alternatives depending on what material you are working your decoupage art on. 
And often based on the shape of your object also. Like, Epoxy resin isn't a good option in case of curved surfaces.

For coasters made of wood/mdf/acrylic, you may want to use Liquitex's Matte Varnish to give it a non-shiny, matte finish.

On glass bottles and decoration objects, you may want to use DecoArt's Triple Thick Gloss Glaze to give it a shiny glossy finish.

More suggestions would include:


water based: MINWAX's Polycrylic
oil based: MINWAX's Polyurethane