In much the same manner as super heated air yields moisture in your air compressor once it hits the cool tank/piping, a mix of filler creates heat as well, and likely will experience a slight amount of condensate if the body panel is somewhat cool. Some eliminate this issue by preheating the panels with infrared heaters, some don't worry about it if the temp in the shop is well toward 100 degrees, some don't know otherwise, some don't care. A nice humid environment may play a part in adding moisture as the filler is mixed. Multiple, thinner layers of filler should minimize the issue vs. applying too thickly all at once, which generates heat nicely. Some rather have the epoxy on first to seal any moisture from the metal.
A widely debated topic, which will never have only one accepted answer. I will say I have seen the video before as it has been out for quite some time, and I have also heard there were prep inconsistencies. I would have rather also seen a head to head using all the major epoxies to see if it was just a DPLF issue. If House of Kolor still made their EP-2 formula with zinc chromate (thanks EPA) I would stack it against anything. That was some good schtuff.
