I have had good results using Krylon Cover MAXX - Red Oxide Primer... Very flat, almost like a chalk paint finish. On all recent buildings I go back with the track painting markers weathering colors and use rust and rail tie brown and hit about 20 % of the bricks... On larger buildings where that would take too much time, I have just used some craft paint colors, some in the rusty orange family and one that's darker and just randomly darkened bricks by stamping the brush to create blotches.... Then seal it with flat clear coat before applying mortar. My preferred mortar method now is the lightweight fast drying spackling.... I make up two batches one tinted darker than the other with india ink... some photos of results below
