Regarding rust and Remington 870 Express guns:
What appears to have happened on some runs of this gun was they did not get a good final neutralizing bath after that grey finish. The answer is simple. Mix Bicarbonate of Soda (baking soda) and water. Degrease the shotgun. Wipe the entire surface with the soda/water mix. This will kill any of the action. Dry the gun well with a hair dryer and oil with any good oil like Breakfree or Slip2000.