just curious

whats better to use mylar or just simple old white paint.

depends on if you are talking about cost or reflective capability. Flat white paint is 85% reflective hile Mylar is 95% reflective. White paint doesn't affect heat retention while Mylar reflects the heat.

If you do the white paint, use flat white.

Vi

Flat white paint is the absolute best for reflecting light

I like to use large white plastic party tablecloths. These are thin plastic with a very white matte like finish I think works very well on walls or hung to get closer to plants to maximize reflective light.
They are about a buck a piece at party or dollar stores...