Initially, you will want to get all of your components in good order. We'll start by making sure all of your gears (the larger ones) and other components look like they're straight out of Victorian-Era mechanics!
Get yourself , your gears, and any other nicknacks you'd like to add for aesthetics to a well ventilated area. If you're quite keen on making everything look absolutely perfect, you'll want to prime all of the items you plan to paint. Usually, I'm big on this, but the end of the summer is coming fast, and I want to make sure this is done for my brother in time, so I skipped the priming.
Once primed, or otherwise, take your gold/brass/copper spray paint and have a go at all of your gears and whatever other components you want in these metalics. Keep in mind, you'll want some varience. I used gold for all of my large gears, but I left the shafts iron, and added some copper and brass later for good measure.
Don't worry about the gears effectiveness after painting. Unless you put on a very heavy coat, the gears will work just as well once painted.