Guns tend to rust under the grips as moisture and sweat seep under and is trapped by the tight fit.

To prevent this apply a medium-heavy coat of wax, like Johnson's Paste Wax or the expensive but excellent Renaissance Museum Wax to the entire frame area that's covered by the grips.
Let dry 20 minutes or so, DO NOT wipe off. Just remount the grips.
The wax will seal off the metal and prevent moisture from rusting the steel.
If the grips are wood you can also wax the inside of them too, but not the rubber types.