Step #1: Pick a basecoat.
The white piece shown here is actually from the back of the robot. It was 3D-printed here at the MAKE office.
I painted the robot bright red, to mimic our Maker Faire robot mascot. I choose Krylon's Fusion brand, with the right red I was looking for. It dries pretty quickly and adheres to plastic really well. I skipped the priming process, but you can put this paint over primer easily enough.
Step #2: Pick a metallic paint for worn edges.
I like Citadel's metallic paints but any hobby metallic paint will do. Citadel's "Chainmail" is my favorite, and it's quite brilliant.
It's pretty easy to get basic weathering effects, as you can see. There are more degrees of weathering you can expand upon: pigments, rusting agents, and other cool techniques are out there to find and invent!