Herc Driver and Miles: thanks guys. The auction building was painted with Floquil Reefer Orange. Once the paint had thoroughly hardened, I used a rag over the tip of my finger to apply drywall mud (I use the ready-to-use kind) over all of the brickwork. After that had dried, I used a clean, dry rag to wipe down the entire structure, leaving the dried plaster only in the mortar joints and low-lying areas. A small amount will also be trapped in the rough surface of the flat paint, toning down that bright orange colour a bit. Then, using a soft 1" brush, I applied a thin black wash: no particular formula other than some Polly Scale paint, lots of water, and a couple drops of dish detergent. I put a generous amount at the top of the walls and let it run down, making sure to cover all of the wall surfaces. Don't brush any more than necessary, or you'll remove the "mortar" from between the bricks. I usually have the structure sitting on several sheets of newspaper to soak up the excess wash, and I also use paper towels to ensure that not too much collects around the window sills, or in the joints between the walls and the pilasters: just a touch with the corner of the towel will suck up any excess. You pretty well have to keep at it with the paper towels until the wash is almost dry, as it continues to flow down the walls as long as it's wet. The next day, after everything was completely dry, I airbrushed a little bit of dirt colour all around the lower part of the walls, to represent splashed up soil (no eavestroughs).
Wayne