The first and easiest solution, (and I bet you learned this now) Don’t borrow out your guns!! I learned the hard way about 6 or 7 years ago I let my brother use my 870 express that was in perfect condition. He took it duck hunting and it rained, he accidentily forgot to take it out of the case and clean it for about a month or so until I found it.
Ok ok, i’ll cut to the choice, I found an gun oil in aerosol called Rusty Duck (??) It worked nicely, didn’t harm the blueing and cleared up a lot of the rust. The gun still has some pits, but after a month in a wet case it wasn’t minor rust my any means.
Please o’ Please don’t use anything abrasive (steel wool) on your gun. If it scratches off the rust, it will damage the blueing as well, and then it will rust way quicker for the rest of it’s life. An oil that is designed for firearms is the best choice, I agree with others that it won’t evap. like wd-40 may.
Just my .02 Good luck hope you can get it cleaned up nice again.