Quote Originally Posted by gainesvillegreen
Flat-white paint is definitely the best of the two. Mylar gets dirty quickly and loses its reflectivity when dirty. As already said, it also loses its reflectivity as you clean it. Flat-white paint does not need to be cleaned except once in a blue moon and cleans up very easily.

As a recommendation though, use panda film! The stuff is extremely durable, highly reflective, waterproof, etc. etc. It is black one side, white on the other (hence the clever name), and is great for keeping light in where you want it and light out where you don't.
Apreciated!