M2 and CPM M4 are not "carbon steels". 1075 on opinel, Fred Perrin customs knives or 1095 on old Schrade or ESEE knives are Carbon steel and getting a patina with a blink of the eye.
Here is a patina on a 1095 Newt Livesay NRG:
I love carbon steels and I love to get patina on them. M2HSS and CPM M4 are totally different animals.
Here is a picture showing a BM AFCK HS with M2HSS blade
and the Spyderco Gayle Bradley with CPM M4 !
Each steel is very different and is not made through the same process. (One is a CPM !)
Two knives that I really love as they give the most modern design with a non stainless blade and M2 & CPM M4 makes great knives for whittling. (it cuts and leaves some kind of "patina" on wood. The best woodchisels are made in M2HSS BTW... there is perhaps a reason.)
Both has been used as EDC without the purpose to get a patina.
The M2 got its patina very quick and I was not able to get rust pots on it (even used on sea shores).
CPM M4 is not getting patina very quick. I was not able to catch the blueish reflection on the blade but despite two little brown spots there is no real patina... yet.
All my M2 knives developed a patina in a week of use.
I got my M4 GB for two months and use it daily and if it doesn't shine anymore like a CPM30V blade, it is not yet dull grey.
I clean it with hot water and wipe it. That's all.
Someone else on the forum has shown a picture of a GB after all a night in mustard. The patina was beautifully dull grey.
Now I don't feel like "forcing" the patina" as CPM M4 looks very stain resistant anyway and I'm not considering to leave the blade all night long with acidic goo on it.
I have even started a BG Patina Thread for comparaison purpose but it's now in oblivion...
With the GB, I have been cutting some sticks of wood all this morning and trimming some plants. The picture was taken after I clean the blade and strop it five times on my leather belt.
Mr Bradley and the Glesser (father & son) were clever enough to offer us a very thin shallow grin with a extraordinary steel which can stand that configuration. My GB cuts hard wood much deeper and easier than my good old AFCK.
cheers
Nemo