Dura Coat makes a compound just for this.
A guy found a .22 single action pistol in a ditch and turned it into the Marshals office. They ran the numbers on it and it came back clean.
It had been there awhile and it was really pitted up on the bottom side. The Marshal asked me if I could clean it up.
So, I took it home, took it apart and sand blasted it. I ordered the stuff from DuraCoat and applied it on the gun,using a plastic knife to go over it. Let it dry, sand it a bit and then I Duracoated the gun with Matte black.
The gun looked new. The pitting was gone and it looked like a it was bought yesterday. I took it to the Marshals office and he freaked out. He thought I was playing a joke on him, having replaced it with a new gun until I convinced him that it was the same gun by showing him the serial number.
That stuff worked great. Its really the only way to get rid of the pitting and the good thing about DuraCoat is that it makes gun virtually rust free.
Its worth trying.
AR CHL inst.