FWIW - when applying paint that is to represent "roof cement" I apply it with a brush and move the brush in ways that simulate the way that roof cement is applied on the prototype (using a mop instead of a brush, roller, or sprayed). I don't like to get it too noticeable but I don't want it to be smooth nor linear. One way I get this effect is to 'push' the brush instead of the normal brush strokes. And also using side to side motions as well. I like to think of the way that the janitors in my grade school mopped the floor.
