I'm way late to the party and signed up just to respond (I have a different 3D writer, but I wanted to contribute something I didn't see already posted (or maybe it was just in German).
I have had similar problems and the issue seems to be dirt or some other material that prevents the surface from wetting properly. The coating seems to brush on OK, but then patches thin as the resin retreat from these regions, and the spots appear. Its seems the surface tension of the XTC-3D is exceeding the affinity between the XTC-3D and the surface being coated. Same reason oily spots on a glass surface prevent water from forming a smooth sheet. Or spray paint fails to coat dirty patches.
The answer in all these cases is the same. Clean the surface. Use a cleaner or solvent compatible with the surface being treated. Deodorized mineral spirits (mineral spirits minus toluene and other aromatic hydrocarbons) work well for many surfaces. One possible solution, though I haven't had an opportunity to try it since I heard about it, is to thin the XTC-3D with acetone or denatured alcohol. These should lower the surface tension and increase surface penetration. And of course, use appropriate safety precautions regardless of what you do. At the least, use gloves with epoxy as exposure to it can sensitize resulting in nasty allergic reactions on re-exposure.
I hope this helps! And thanks to those posting on solvent polishing PLA with acetone. I'd always failed with it (and with THF polishing of PLA) but I learned some things I'm going to have to try out with selected brands of PLA.