I found this to be VERY true myself with mylar. I bought a cool tube and once I installed it and brought the 1kw down closer to the plants the mylar FRIED the area to over 90 degrees. I have a little temp gun and was checking temps of different things regularly. With the temp gun I discovered the temp of the light in my cool tube was 85-130 degrees depending on where at the light I pointed it. Temp of the MYLAR two feet away......93 degrees. I painted regular flat white paint (2 coats) over all my mylar and retook temps at the exact same specs (same distance/light in same place). The temp of the flat white paint was 80 degrees.... a HUGE difference.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.