Epoxies that we use do not "dry" but cure as a result of a chemical reaction between the resin and the hardener. you can pour the epoxy two inches thick and it will cure fully as long as you mixed it thoroughly and had the correct mix ratio.
Thinning it to spray will in no way make it cure harder, or be any better if you have to do many coats as opposed to one thick one. I know this information from speaking directly with the manufacturers of the Nu-Lustre 55 products.
In fact if you allow the thin coats to cure fully before respraying, you will not have as secure a bond as with one thick coat because the layers will not chemically bond, only mechanically.