I don't understand the worry about CPM-M4 rusting. I've used a CPM-M4 Military and Gayle Bradley for the better part of 2 years now as part of my EDC lineup, and the GB had yet to develop a noticeable patina. I forced a patina on the M4 Military, but that was just purely for aesthetics.
A couple of nights ago, I experimented with Muriatic acid and decided to etch some blades. I did the Air, Gayle Bradley, a PPT, and a Kershaw Rake. The GB's CPM-M4 blade was left in the acid for 30 minutes, and came out still looking satiny, with some darkening. The Air had its blade hung above the acid along with the GB liners, just getting the fumes to do the etching, and received a lighter patina while hanging for about an hour. The stainless liners of the GB turned a little dull, which is what I was after. They seemed to be more affected by the fumes more than the Air's blade. The Kershaw Rake with the composite blade didn't do so well, and there was pitting on the 14C28N spine, but the CPM-D2 edge remained unaffected. I had to take it back to the grinder and re-polish the blade. The PPT's S30V greyed and dulled after 2 hours in the soak, which is what I wanted. For a non-stainless that's supposedly very prone to rusting, I think CPM-M4 held up surprisingly well in the etching solution. The stainless surgical forceps I used to remove the parts from the etching solution, despite being constantly rinsed after being in the solution, got dull and was noticeably etched more than the carbon steels.
