What if you painted the sheet with the mortar color first (as Ralph mentioned) and then used a foam roller to paint the brick color. I’ve used foam roller covers to paint smooth surfaces before, although I’ve never used them for hobby type painting. The foam rollers have no nap to them, so maybe they wouldn’t blotch the mortar color. You can find them at most hardware stores- Home D, Lowes, etc. Just a thought. Oh, and by the way, the JTT sheets look real nice. Thanks for the comparison shots.
-Kevin.