Not sure about the paint on the upper but I stick my Bolt Carrier and Bolt inside the ultra sonic cleaner with some 40:1 diluted Hornady Brass Cleaner and Distilled Water and it does a pretty good job of cleaning the Bolt Carrier Group, firing pin, and other small parts from the AR15.
I do worry about the finish on the bolt and Bolt Carrier though. I'm not sure what type of factory finish they put on the metal of the Bolt and Bolt Carrier but when it comes out of the ultra sonic cleaner it's dull looking and brassy looking in color. The bolt is clean though.
I sent a email or message to Ambush Firearms Warranty Department asking about this as I found a chip inside the upper receiver part of my Ambush Firearms. It looks like a small piece of the aluminum has chipped off the upper chamber area. The Bolt Carries slides by this area of the upper chamber.
I know that the Barrels metal is finished with a Salt Bath Nitride type solution.