Epoxy resin is what I prefer to use to finish most of my paintings because I absolutely love the look, but it's definitely not a must. You can use any type of finish you like, for example, spray varnish, brush-on varnish, etc. Varnishes are available in different sheens - matte, satin, and glossy. Unlike resin, varnish creates a very thin coating on your painting. You can find an example of varnish finished painting at the end of this video.