The most important step with any painting is the PREP. You've got to make sure to get ALL of the grease and oil off of your firearm. Brake cleaner works, as does Gun Scrubber, as does boiling the parts in water. The rule of thumb for me is: Once you've got it where you think it's totally de-greased, go over it one last time just to make sure. Any residual oil will mess up your paint job. By that same token, once you've got your firearm de-greased, only wear clean gloves--the oils from your hand can get on the metal or plastic and mess up the paint.
Tape off any areas that you don't want painted. Take a foam earplug (the cheap ones, the ones people use at the shooting range) and put one about 3/4 of the way down the muzzle so paint doesn't get into the bore. If you're painting a firearm with an open ejection port (FAL, AK, etc, etc) put crumpled-up newspaper (or any paper) into the receiver and LIGHTLY pushed into the chamber, then tape over it. This makes sure no paint gets into the receiver or chamber. Honestly, even with an AR I'd close the dust cover and still put some crumpled-up paper inside the receiver to make sure no paint got in the chamber. Some people will cover their serial number or brand logo with tape--that's totally a personal thing and is up to you. One thing I suggest, though, is to use the Blue Masking Tape (that you can get wherever paint is sold). It sticks well, comes off easy, and doesn't leave behind any sticky residue.
I personally used Duracoat clear coat over the regular spray paint. It came out well, and provides a very durable finish to coat the paint. However, it may not work in every situation, or for every one. Duracoat is EXTREMELY durable. It's hard to get off. If you are going to change your camo for any reason down the road, I wouldn't use Duracoat clear coat. It almost impossible to get off without sand-blasting. If there's a chance that you'll want to change the camo, use regular spray-on Matte Clear spray paint that you can get at any store that sells spray paints (Lowe's, Home Depot, Wal-Mart, local paint stores, etc.) It'll do the job of protecting the camo from rubbing off; but isn't as permanent as Duracoat.
The final thing is to make sure you let the paint cure properly. If you're using regular spray paints only (including regular spray-on Matte Clear coat), once you finish painting set the whole thing aside for week. If you're using any Duracoat (even if it's just Duracoat clear coat over regular paint), set it aside for 4 (four) weeks to make sure it's totally cured. If you mess around with it before the paint is totally cured, you have a good chance of chipping the paint easily. I'd follow the above cure times even if I was putting part(s) in the oven, just to make sure it's fully cured. (But then again, I'm pretty obsessive about it
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Hope these little tips help out. Make sure you post photos when you're finished!