Is this a simple clean/fix or a bigger issue?

[My1stGlock 144]

So my buddy has a stainless steel 1911 and I noticed what looked like pitting or carbon build up or dirt on the lower frame but then when I ran my finger over it it was not rough. He's cleaned it before but not sure what he's been using to clean it. Is there something that someone can recommend that he could possibly try? Pic is attached. 

[robot_hell 72]

Because it's stainless steel, you can try a lot of different solvents to take it off, as long as they are not abrasive. Start with CLP, if no luck, move to powder solvent like Hoppes 9, then maybe acetone. Only then break out the big guns... If chlorinated brake cleaner doesn't take it off, nothing will.

[My1stGlock 144]

Any recommendations on the type of material he could use to scrub it with or just spray it down and let the cleaner work into it and then wipe with a cotton cloth?

[Zeke 5,504]

11 hours ago, My1stGlock said:

So my buddy has a stainless steel 1911 and I noticed what looked like pitting or carbon build up or dirt on the lower frame but then when I ran my finger over it it was not rough. He's cleaned it before but not sure what he's been using to clean it. Is there something that someone can recommend that he could possibly try? Pic is attached. 

That appears to be oxidation.

Nu finish car wax. Wd 40

[My1stGlock 144]

46 minutes ago, Zeke said:

That appears to be oxidation.

Nu finish car wax. Wd 40

That's odd. He cleans the gun and keeps it in a silicone treated case. What could be causing it to oxidize?