Hi mate, sometimes the screws holding in the pad stops are badly corroded and are very difficult to remove. If the screw snaps then the calliper is finished so watch and not break it in.
The calliper will not rust but oxidization of the calliper under the pad stop may cause the plate to rise slightly causing the pad to become a tight fit.
Perhaps a slight tap with a small light hammer would reinstate the plate to its original position.
Buffing off a very light edge from the pad top and bottom area where you are having the issue is common practice within the motortrade.
Refurbing or replacing the calliper is an extreme measure and a cost that is not really required mate.
But if you want an exchange calliper from me then these are available next day, but best replaced in pairs.
I assume we are talking front callipers here............
Let me know how you get on.
Cheers,
Alex.