what Petah said but there are some things you can do that does little to hurt the value of a gun, for instance an old military rifle with an oiled stock that's heavily scratched and dinged. you can take super fine sand paper to it to t get rid of the top layer of oil and dirt/crud, superficial scratches, and make the more serious ones less visible and then using the same type of oil originally used such as linseed oil and it will do very little to nothing to hurt value while on the other hand completely sanding down to get rid on all scratches and gauges, restaining the wood and then giving it a clear coat finish will severely injure the value.
ignore my complete lack of capitalization. I still have no problem correcting your grammar.
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