Perfectly flat aluminum foil is 85$ reflected, and perfect flat mylar is 95%
we all know thats pretty damn hard to do and using alight meter i found you
will get closer to 65% for foil and 75% for mylar.
Hot spots on foil dont happen unless you ball that shit up. Tested that theory with 120*F heaters.
Mylar got hotter behind it, this is probably because it holds the UV spectrum where as aluminum reflected it.
If your aluminum is more then a foot from your plant LOLWUT at hotspots ON the plants, not going to happen.
Mylar also caught fire when i put it against the bulb
Aluminum did not melt or catch fire. and i was able to still hold it. "test for yourself, put a flame to it, then hold it"
Flat white is 60% reflective....
Now so your best bet, based on the fact you will never get aluminum 100% perfectly flat.
Mylar -> White Paint -> Foil
Taking cost into acount.
White Paint -> Foil -> Mylar
Taking both cost and reflectivity
White paint -> Mylar -> Foil
In almost all cases, all 3 are viable, and you must consider what size your space is, and what size your wallet is.
And if you think at any time someone may visit before plants are done, just use white paint. simple