There are a couple things that can cause waves. One is warming the resin up too much before you apply it. The other is torching or overheating the resin after application. When resin gets warmer it gets thinner, so if you get it too warm it doesn't have the viscosity to have a thick smooth build. <-this usually happens with a butane torch or heat gun because they don’t pop the bubbles so you end up holding the heat on your project too long. As soon as you apply the resin do a quick pass with the big torch to remove bubbles, wait about 5 minutes and torch again.
Humidity can cause several issues from a wavy finish to dimples. Another thing to mention is the thickness, too thin or too thick of a layer can cause it to be wavy. And for Fast Set specifically, if that's what you're using...it doesn't always have as smooth a finish as Artist Resins. The reason is that it starts curing so quickly and getting thick that it doesn't have time to self-level like resins with a longer cure time. So you want to really rub it in with your fingers, not pour it on and "float" your fingers across the surface. That would leave deeper areas in the layer that won't have time to self-level. Then your final coats of Artist Resin will self-level and give you that glass finish. :)