A couple thoughts: You could also look at a tennifer type finish for the new button. Pretty rust resistant and might look original, if you care. You could also look into getting Birdsong Black T applied to the new button. It is quite durable.

If sweat is causing your problems, rather than just the humidity, consider switching to horsehide or kydex holster. Cowhide absorbs much more moisture and will carry that over to the gun.