for rusty dumpsters, here's what I did to mine :
paint the whole thing in a rusty colour (you could easily use valejo terracota or some other red brown maybe even with a drop of yellow added to it)
then I use liquid mask. I apply it to the model with a sponge (one with fairly large gaps, not the stuff from blisters)
when dry I airbrush the colour I want the dumpster to have over it, but you could also use a brush (probably best to dab the paint on as strokes could tear the mask off)
Once that is dry, I rub off the mask et voila :
P.S. since I made these I discovered the dust and rust set from Lifecolor. Awesome paints