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my 590 is already park'd so i'm not sure why you're asking about that.  Is there bare metal showing where you've scrapped off rust?  

I had mine sitting in a closet where my a/c ducts run through for 2 years with only an occasional cleaning and lubing with no problem of rusting.  

Maybe you should try keeping a good  film of CLP on it and if you are going to lean it against a wall try another one.

I have (had) both a blued and a park'd 590.  (I say had because when they were recently refinished after a hosue fire, the gunsmith misunderstood my instructions and blued them both instead of park'ing one of them.)

Anyway, the blued gun always rusted at the least little hint of moisture.  I swear that was the thinnest bluing I have EVER seen on a gun.  Eventually it kinda looked like it had spots.

That said, my recommendation is for you to go buy a $50 Duracoat EZ-Finishing kit and Duracoat the gun yourself.  The kit containes everything you need to do 2 long guns including the Duracoat, reducer, degreaser spray, airbrush setup and an instructional DVD.  The only bad thing is that I haven't had good luck getting a DVD that I can actually watch from start to finish as they have some sort of formatting problem.  Never fear, they're working on a good old VHS instructional film as we speak.

Duracoat.  That's what I would, and am going to, do.  You can even choose what color you desire.

Duracoat

Eric