Yes, Kroil has a very high "creep" factor.  Keep it away from ammo.

Your GI mags were originally blued, which is a form of rust.

Anything that removes rust may also damage the bluing.

Rather than use harsh abrasives, here's a couple of things to try.

No guarantees on possible damage to original finish, though.

First, get one of those hard, grey erasers, maybe from a business supply store, and remove the rust spots one-by-one.  Touch-up with cold blue, after de-greasing.

You could also try dropping the de-greased body of the mag into fresh, straight-out-of-the-bottle Coca-Cola for a while.  That'll remove light rust.   Naval Jelly does the same thing, but it is much stronger.  Cold-blue the former rust spots.

To protect the mags, try a good brand of auto wax. and re-apply frequently.