Oil slick is an anno, it's not painted. You gun is also not painted, it's annodized.
DON'T SPRAY PAINT A GUN. It will chip, and will look like crap.
The cheapest "re-coloring" techniques is to contact KPcustoms and have powder coating done. Powder coat is not paint, but is a statically applied pigment that's actually HARDER than anno. It's also no thicker than anno at anno's top range(ie CHEAP anno), and you can hit the crap with a hammer and not chip it.... He does basic colors for 35$ for up to 6 pieces, and it can be done to any metal that is a conducter.
anno will run 65$+ for a gun, and isn't as durable, but offers more coloring options. It is an aluminum only process.
One of the benefits of your dad working at a sheetmetal shop is you get free powdercoating. So far I've had a high-heat silver, flat black, gloss black, like a baby blue, and my dad said they do red sometimes.thumup: