I'll find a better product later, but reading this thread just got me to coat the metal of my M&P with a light sheen of Turtle Car wax while I sat here.
I suggest flat toothpicks with the ends sliced up to get the grooves clean of wax buildup.
I have had a small 1/2" stripe of rust appear on one groove of the barrel, cleared up with a brush, some oil, and about a quarter hour of mild profanity. I'm hoping it was a careless contact with the bathroom floor after a shower.
I'll keep my eyes peeled for any spots.
Conditions, IWB, Constant carry, no undershirt but no direct metal-to-skin contact. If I'm wearing pants, It's on. If I'm not clothed , it's on the belt and draped on a chair near, or on the safe next to the bed.