+1
Bad finishes can hide a lot.
I recently bought a used Wingmaster police shotgun that had a bad parkerizing job. My local refinisher took one look at it and recommended we not try to polish/reblue it because it was so badly pitted. I didn't notice the pitting under the parkerizing, but he took a small area down to bare metal on a heavy duty buffer and showed it to me. Instead we decided on a bead blasting to get the metal really clean, then a rough blue and a parkerizing.
Regardless, the external finish has no effect on the overall functioning of the shotgun. Internally it was just fine. That's what is great about the 870!