Mold??? thats a new one to me!
maybe the wood has a living fungus within that was not noticed during harvest??
I managed a old rooming house 4 years ago, it had 4 washrooms all lined in wood and all coated in mold, the only thing that killed it was bleach and water used over a period of a few cleanings....
You might want to strip the finish, bleach the wood with bleach and water let it dry and then do it again a few times.
Then restain and oil treat the wood..
Aside from that sir I have ZIPPO idea how to kill the fungus while preserving the present finish.
If you decide to go the bleach and water route make shure you do the inside of the martinis stoch "where the locking bolt is" just in case theres thingys living in there.
Also ....could there be a fungus or mold infestation somewhere in your gun locker?? Maybe your getting contact infection from a outside source?
Mold is bad for stock wood but also for you and your familys health.
I hope any of this rambling is of any help!!
Cheers
