I've used the Doc Obrien's from Micro-Mark for years. I like them. Certain colors, like the grays and white,
really dig in and work well, as do most of the rust red, and brown shades. The black is one I could never get
to stick well, even when grinding them in with a brush.
Most of the steam loco weathering I've been doing for that past few years has been with those powders.
Since the light colors adhere well, they work well on slippery plastic truck sideframes.