I'm beginning to populate my lab scene this week, and the first addition is a 7 foot rusty gear...of doom!
It's made of scrap plywood (I would have used foam, but there's about 20 sheets of used plywood laying around). Base coat of grey, smeared on coat of brown tempra, then sea sponged/mottled brick red, terra cotta, dark orange, and metallic bronze acrylics.
I wanted it to be twice as big, but the garage is only so large...
Here's the plan for the whole scene, which should explain why it's only a partial gear (of doom):