Refinish or leave it be? Rail wear

Search online for what well used service M9's look like. Much the same for an M16 carried by a villager in the mountains of Vietnam over 45 years at the time the photos were taken.

Anodizing is a surface finish treatment and doesn't have much to add to the strength and longevity of a part other than to slow down electrolysis. Consider this: worst case scenario and one that is potentially life threatening to an entire family - the alloy rims bolted to the cast iron rotors on the four corners of your family car. Even someone's personal truck - often no anodizing at all, certainly no grease, no periodic maintenance of the mating surfaces for 40-60,000 mile until a brake job. Not even an annual torque check on the lug nuts.

There is the same concern for using the milspec lube on the barrel nut to upper receiver on an M16 - yet one of those may not see an arsenal rebuild for 20 years, with all sorts of training, service in a combat zone, etc and likely some poor maintenance too boot. That nut is never removed unless there is major damage. I'm not aware of any servicing requirements altho the interwebz get a lot of posts over what grease to use building one the first time.

Anodizing can be redone, it's a batch process, it will restore a finish, but there is no guarantee it will result in a matching finish to another part. Those of us who buy lowers and uppers from different sources are familiar with it, and AR history fans can sometimes list the models that all came with mismatched finishes. In the day, M16's were originally grey, not black - it's just the amount of dye in the processing that does it, and controllling the inclusion is very tricky. That's why stripped lowers and uppers can be requested in matching finishes - pulled off the shelf different batches they may not be very close.

And after its reanodized it will start wearing off again. Slight amounts of dust - which can rate as hard as diamonds - will start the process all over again. Cerakote is good but only slows the process - if it's done right. One Marine contract for a fancy 1911 had hundreds returned, which was a historic sell off when they hit the market.

Oh the hurt feelz when the box is opened and they parts aren't identical. It's a decor faux pas for some where the military gives it a very low priority. Even matched they won't stay looking nice thru an actual deployment. They may even get spray painted in Coyote and some other color, then let it wear down back to black on the highlights. Army issued guidance to do that years back. I did a 6.8SPC in Foliage Green and its finally looking good. The matte clear on top is even giving it a yellow tint where I hold it.

Everything turns to rust eventually.