I found a pair of really ugly toy ray-guns a while back and I decided to try out a new weathering technique on them. I think they turned out pretty nice.
four step process;
1. spray metallic (enamel)
2. brush in rust colored enamel
3 brush on acrylic flat black
4. while black is still wet, dab with crumpled newspaper.
Repeat for extreme awesomeness.
Be sure to clearcoat after paint completely dries. Acrylic is not waterproof and chips very easily when not protected.