Da40kOrks asked me about weathering his wheels on his flakktruck with an airbrush to give it a ground color he uses on his ... This is how I'd do it.
Buy Weathering Powders (Affiliate links):
ScaleModelMadman's Weathering with
by Jocelyn D.
by Talisa Tossell
by DIY UH ART AND CRAFT
by Ferry Creative Gallery
by Stitching Class
by Baylee Jae