Paul,
If the surface isn't smooth enough, hence all the talk of gloss finish, the decal won't "stick" to the surface, on a flat type of finish the decal is essentially resting on the "peaks" of the surface. You can paint and morter your bricks, I'm guessing you are going to have to paint where you would like to place the decal with gloss and then go over it with flat and finally weathering. When the decal floats off the backing it shouldn't carry anything with it that kept it affixed.
The only time I've ever used glue has been to stick paper signs to buildings for replicating painted bricks.
