While I know there are hundreds of posts about rust, I'm asking for specific advice here. The only rusty part of my 83 Miyata 610 is the chainstay (SEE PIC). Must have had some major chain-slap going on! The rest of the bike (including insides of tubes) looks pretty good, so I was not thinking about the oxalic acid "bath" I've read about here (but have never tried).

If you have had experience with a localized problem like this, I'd appreciate any advice---both how to treat the rust, and what to do with the surface after. I am not seeking a museum-quality restoration here, just protection and good-looking, functional approach.

THANKS!

Honestly, for small patches of rust like that I just scrub with WD-40 and a brass brush. Touch up paint once done is good. Sounds like a good looking bike!

as said above.

instead of touch-up paint, you can also use clear nail polish and wax. may or may not look better but should keep it safe from rust.

You'll need to get all of the WD-40 off before you touch it up. No paint sticks to an oily surface.

If it were my bike I'd likely just work some #0000 steel or bronze wool on the area with WD-40, clean thoroughly, wax and install a new rubber slap guard. You could use touch up paint before the wax step. To me, however, a bad touch-up job or color match is worse than a cleaned and waxed area without paint.