i am a little confused as to what you are tring to do... are you trying to paint a molded brick wall, or put a stencil on a wall to simulate brick? To paint a molded brick wall with mortar lines, I first paint the brick color, then thin some light gray and slop it over the bricks - wait a minute, then wipe the surface using a paper towel, leaving the gray paint in the cracks to simulate mortar. If you just want to put a brick pattern on a flat wall, i suggest printing sheets of brick pattern using a computer, then pasting them to the wall.
kevin