“Refinishing” is a good one that I had forgotten.
As far as “renovation” and “restoration” are concerned, I always feel that renovation ( = rejuvenation?) should be a much less invasive process than restoration, probably involving nothing more than a general clean-up, removal of surface rust, replacing seals, lubricating etc. Nothing that ought to offend the purist.
Restoration, on the other hand, is a more thorny question. Most collectors on here are very vocal about their dislike of refinished vintage guns. Is that only when the new finish is obviously wrong, or would even a refinish that perfectly matched the factory original still be frowned upon?
It strikes me that there are two types of collectors. The first are represented by collectors of vintage cars or old-master paintings. In both cases, restoration is perfectly acceptable, and in fact is deemed desirable when needed. Then you have collectors of porcelain and china, where any form of restoration is regarded as a no-no, and can have a disastrous effect on value. Do the majority of us airgun collectors really fall into the second camp? Are we snobs rather than realists?