Likely, your gun had a "blue" finish on it when it was new and the "blue" finish (a controlled rusting) is rusting to "red rust" brown. Air with moisture and oxygen does that.
***YOU HAVE TO MAKE THE CALL*** If you refinish, you destroy the guns "originalness." I have no idea how well Marlins are collected around you, not the greatest anywhere--all Colt & Winchester; how much do you want to preserve...
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Friend, buddy had a 73 Win. "in the family." It had been refiinished. Collector told him it would have more value with the new finish removed, "in the white," than with the refinish. It was a good job...
Tricks begin with a plain old wooden pencil. "SOFT" lead (which is really graphite and clay). You rub the spot with the lead and remove the lead and rust with the plain, pink (or green) rubber eraser.. Quick, easy, BUT: #1). I did not say and DO NOT use "ink" erasers with grit in them. Like sandpaper. #2). With the aged condition of the rust... it will be quicker and easier to "go thru" /penetrate so be careful. BE CAREFUL. Start with a #2 and move as needed... I think #3 is softer... Actually leaving the graphite is a poor rust preventative but better than nothing and when it rubs off on your clothes, you will hear from the laundressss....
If you are going to use a "wool" there is brass wool available which is softer and more forgiving. Most large hardware stores will have it if you ask. Steel wool is harsher and if you are not a "master craftsman" --I use brass wool.
The purpose of finish is to cover the steel with an acceptable appearance and a layer of holes, pores that will hold a protectant. Animals fats and mineral oils are the common lubes from that era. Whale oil, too. Outlawed today. GOOD NEWS: Today we have so much improved lubes beginning with WD 40, which is now CRUDE; you can move on, stop the deterioration, use the gun on occassion... Break free, Triflo, vist a hardware store/auto store and read labels. I would not mix them, but they are there and a blessing/BLESSING compared to lard, tallow, rosin, etc. Study the labels. GUNK will remove blue... !! (Learned that the hard way). enjoy your prize. luck.