Undercoat must be dry or it could come off. Have used thinned Micheals,Hobby Lobby, Walmart, acrylic "squirt" paints for the motar and mixed in small washed out old plastic coleslaw, butter, etc food containers. Slop on, we use a small like shaving brush but anything would work, and don't wait too long or you have to rub too hard to remove it off the brick tops. We don't really care if it all comes off either.
Also, the rag you use to wipe off the excess cannot reach into the crevices.
For example old t-shirts work way better than insde out old white socks where the pile cleans out the mortar grooves. Shallow mortar lines make it even harder. Work gently:-)
