I had good results refinishing one using Duracoat

No idea if powder coating would be better, as it seems to have some drawbacks.
Maybe see if anyone else has powdercoated a gun & added the video of doing so to YT

I think this would be the biggest issue. If you end up taping off areas, you will end up with a way for corrosion to get up under the powder coating and further degrade things.

Cerakote is a better and thinner alternative.
On the other hand, if it is a really beat up gun that you really just want for knock around, leave in the barn duties, you could sand it down and use rustoleum on it.

I used Rustoleum on mine.   It was one of the 870s that came with a really rough finish,  so it sticks great.

If you can powder coat, you can use Norrell's Moly Resin, and I would recommend it for this application. It's a thicker coating than Cerakote, but *way* thinner than powdercoating, and it's tough as nails. It will also remain matte longer than Cerakote will over time with rubbing/wear.

Check it out...

Prep the part like you would powdercoating, heat the part to 180' F, spray, then bake at 300' F for one hour.

Rustoleum Appliance epoxy enamel. Best spray on coating I've ever used.

Coincidentally, I just did a rusty extension tube on my 870 the other day.

The trick is to cook your gun for 450 for an hour or until the paint goes satin. It's almost like magic. The finish THINS out to a beautiful, smooth sheen, and becomes hard as a rock. Doesn't even look like paint.

Try it out on a piece of scrap, if you don't believe me. A can is maybe $6...

450 is way too high, you're getting into the heat range where you'll be altering the tempering of the steel, stick with 300 degrees.

I used Duplicolor engine enamel with ceramic a few years ago on a beater 870. baked in my Traeger at 300 for about an hour. finish has held up great. It smelled like bacon for a year though.

When I picked up a cheap 870 Express for 3gun and discovered that it developed surface rust the just on the drive home from VA humidity, I ended up giving it a couple of light coats of Krylon. It's been 12 years and that coat is still holding up, and personally I think the adhesion and protection seems to be better than similar stuff I did with Dupli-color rattle cans, although the DC stuff seems to he thicker. In short, if all you care about is some degree of looks, perhaps reducing surface glare, and primarily rust protection, I'd Krylon it some flat color and call it a day.

I completely disassembled the gun, hosed off the the outside with brake cleaner, then put paper towels into the cavities, tube ends, and barrel ends. When I needed a fine line some masking tape. I used a cardboard box as a paint booth, and let it dry an hour between coats. Finish ended up super-thin, and super durable once cured.