Patina?

Some time ago, I made a toolchest from pine. The wood was well dry when it was made, and it was finished with a coat of Colron wood stain, and three coats of yacht varnish (as it would eventually reside in a damp garage).
Once made, it actually stayed in the house for about six months.
During this time, all the brassware associated with the inside of the box (screws, hinges, saw backs etc) developed a sticky green film on it's surface. The external brassware didn't.
This film came off easily enough with fine wire wool and WD40, and the brass looked like new again.
Now the box is finally in the garage, and the green film is coming back again.

I don't think it is due to the atmosphere that the box is kept in, as it has happened in both dry and damp conditions.
As I say, the film can be removed, and the brass looks as good as new.

Any of you experts out there got any ideas??
Any help would be appreciated

 

Thanks both for the advice, I will spray with laquer and see how it goes.

Apologies Wolf if it was a stupid question!!!!!