+1
I live in central Missouri, and have and use CPM-M4 almost daily for everything from pocket carry of the Gayle Bradley (nice knife), to woods/camping use of fixed blades, to all out insanity of competitive cutting practice with a large choppers. I can assure you that my knives are exposed to some outrageous materials, and basically I only wipe them down with a dry cloth, and (when I deem necessary) I spray them with silicone lube.
Occasionally I see some light "bloom" or specks of rust on the blade(s), but I find that it generally is removed with additional use. I used my Gayle Bradley as a stirrer for my coffee on a weekend camping trip and intentionally put it away wet (to see how bad it would get). No "rust" but there are some interesting stains on the blade now...I appreciate these as memories of a good trip.
Opinions vary on how much of a problem discoloration and bloom can be (it is a personal decision). I suggest that if you are really using the knife, these things will not be as important as the attributes of the steel (all knives will blemish over time and use via scratches and so forth).
If you demand a pristine blade that looks like it did the day it was made, M4 may or may not be the best choice for you. Some people get a little more diligent with rust/stain prevention than I do, but I suggest that (if you use and carry the knife) the day may come that the knife will see exposure and you will not be able to avoid a stain.
Hope this testimony helps. This knife was one of the most talked about Spydercos for the past few years for good reason. They are really well made, and the steel has some nice attributes.
