Mark, I am in the same position. My approach is not an expert Model "Railroader' or builder. But my knowledge on paints can / might be helpful. It might spring forth new ideas for you or any other reader. I am in means an expert.
My attack plan.. is to use.. 'Craft Paint' as in 'Doc in CT' thread. Doc gave a nice demonstration on doing the mortar. But as far as your case.. and mine.. I figure if I use the Craft paint (water soluble) .. in a water down base.. I should be able to lightly give a mortar.. weathering finish. My approach .. although you didn't .. is to preserve the original finish.. by using Testors Dullcote. Once that is on, as far as I am concerned.. even if I do the Craft paint wrong, I can lightly wipe it off. After I am finished with that particular coat of a watered down mortar finish.. I'd probably put another light coat of Dullcote .. to protect that application. Then if you want to go back and do some more.. or add various colors to give more depth to the bricks.. you can do so.. and if you don't like.. wipe it off lightly. Then when all done.. apply the last finish of Dullcote. One thing I didn't mention, I will 'mask' off the windows..and other things.. that the Dullcote might ruin. As far as masking tape.. I use the green painters tape.. instead of the blue. The green is a finer tape.. with no 'crinkles' like the blue painters tape. One other thing.. Doc in CT thread.. he uses the craft paint .. thick.. and with a jabbing motion.. pushing .. and shoving the paint into the cracks to produce the mortar look. He lets that dry and then rubs it off with a alcohol swab.. very little water.. and erases the excess. I would like to give you a photo.. of one I need to do.. but right now.. I am working on 3 models. Here is my first attempt on mortar.
I am happy.. but I have gone through the whole spectrum of paints.. and found the Craft paint to be the most forgiveable.. and easy clean up. Good luck.
Keith