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If you are talking about a PVD coating it might cost more than a new bolt.

DJM

One thing you might try is a product cal E&L Gun Lube.

It is a spray on graphite product that was originally developed for use in space where liquid lubes will not work.

Once applied it stays on very well. Water will not take it off, but Gunscruber will.

It is a very good lube where a dry lube will work. It is a black color.

I have also used it on rifle and shotgun bolts not only for lube but if they are silver colored, it covers up the shine.

Also dust dirt, sand, etc. do not stick to it.