I am currently using mylar and I think it is creating heat, but not exactly sure though. Mostly because I did not get it put in very smooth. After this grow, I think I will bring the box down and make a few mods to it and paint it. First grow and finding ways things could be better.
I am already wanting to start my second grow, so this first one needs to hurry up. I am dying to try some new ideas during the grow process.
As far as primer goes, I dont see how paint could get any flatter than that.