Ok, here goes. This duplicates a thread on deep cleaning an 870, inside and out, back on TFL.
I sugges this once a year, or every 1K round sor so.
Take it down, including the forearm, action bars,
bolt and trigger group.Unscrew the big nut at the front of the forearm with either a special took available at Brownell's, etc, or use a pair of needletip pliers with the tips stuck in the notches. Clean and relube. Be sure to get the inside of the tube the forearm covers, this is often the place rust starts on pumps. Get the threads, too.
NOTE: All relubing mentioned here is"Wipe on, wipe off". Coat all surfaces with lube, and remove as much as you can with paper towels, rags, etc.
Scrub the bolt and TG with Hoppe's and toothbrush, or use a spray cleaner.In either case, get the cleaner off the parts as best you can and relube.
Go inside the receiver and clean all surfaces, then,you guessed it, relube.
Do the same with the bbl. If there's tubes, unscrew and scrube/lube the threads in the bbl, ditto for the choke tube and bore portion.
Express finishes hold lubes like CLP well. Some of this or SLIP 2000 and a silicone cloth will save you from corrosion under all but the most severe conditions.
Reassemble, then with the action closed, put ONE drop of lube on each action bar right next to the receiver. Pump a few times, throw it to your shoulder, smile and store it until the next time.
HTH....