If you have an AEG w/ a plastic receiver, you need to accept that you will never be able to strip all the paint off, unlike a metal receiver.
Your best bet is to disassemble your AEG, and take some steel wool to each part one at a time. You're not looking to remove all the paint completely, just take off any "texture" left behind by previous coats and to make the surface uniform. Then you'll want to wash these parts off in a sink, let them air dry and reassemble the next day.
Once your AEG is back together, hang it up w/ a wire coathanger and block the muzzle and magwell so paint doesn't get inside. Then spray your AEG w/ thin coats of Krylon Camo Black.
You should end up with something that looks as "stock" as you're likely to get.
Hope that helps.