Turns out, the last time I posted about this was way back in May(!). I started by trying an acrylic wash as I did with Ballantine's, using craft paint this time (instead of Polly Scale paint), and a couple of different application techniques. Unfortunately, none of them really gave me the effect I was looking for, so I put the project on the back burner - for a few months, as it turned out.
I picked the project up again with a "do over" - as in, I just sprayed the castings with a fresh coat of red paint and started again with the experimenting. This time with some spackling...
In some small areas, this ended up being an "ok" technique, but still ended up making the walls much too light with mortar/spackle all over the faces of the brick no matter what I did to try and remove it.
I thought maybe I'd start over yet again, this time spraying a gloss coat over the red - figuring part of my problem was mortar getting caught and drying up in the microscopic "pores" of the flat finish. But while I contemplated that, I decided set the craft paint and spackle aside and try some oil paint...
No gloss coat needed - the oil paint is easy to remove even from a matte finish . . .
I tried it out on a spare chimney (above) and here it is on a section of wall next to the section I did with acrylic wash (oil paint on the left, acrylic on the right, in case you were wondering...)
Other than how easy it was to remove it from the brick faces, the other thing I noticed about the oil paint is that it takes a LOOONG time to dry. The little bit above was still not completely dried through even after a few days.
I also have never worked with oil paint, so am not at all sure how it will react to subsequent weathering/finishing methods (how will alcohol & india ink washes affect it, for example?)
And, in addition to using a gray-er color for the mortar and definitely weathering the walls over all, I'll probably try and highlight/distinguish some of the bricks with these colored pencils.
I still haven't decided whether to continue with this oil paint technique or try something different. In the meantime, I decided to strip the walls I've been practicing on (soaked in Simple Green for a week) and start over.
As they say.... "if at first you don't succeed, try, try (and try) again..."