Hi folks, I sometimes do window painting. I usually bring a gallon of flat
white with me and use a little of the color I want to tint a bit of the white.
Then use the color full strength over the primed part.
ALWAYS use flat paint for best coverage. Semi-gloss will streak and peel of
very easily. If it is a more permanent piece, use flat only for both coats.
For something you want to clean off, flat, then semi-gloss over that. Cleans
off with a razor, and lots of Windex.