I'm rehabbing a new to me 1984 Trek 610. It has cosmetic issues I've been dealing with in the forms of scratches and some light rust. One thing I've never had a lot of luck with is dealing with rust in the top tube cable guides. That's a tough place to get to. Any good ideas?

I've used dremel tool in the past. It works but it will also take a lot of the paint out which I'm trying to avoid. I've got a really good match with touch up paint so I could deal with losing a lot of paint there but I'm hoping that someone here has a better system than this.

Oxalic acid.

That is a tough area even if you completely strip the frame of all paint. The paint and rust sticks to the area inside and at the junction of the Braze on and the tube. I have several hobbies of which one provides me with brass brushes for different caliber tubes. I think I have been using a 22 Caliber brass brush.

Although I haven't tried it, you could put a "dam" around each using a putty of some sort, like plumbers putty and fill with OA. Now that I have verbalized it, I may try it myself! My experience is that OA will not remove or discolor paint.
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Hi. I like Naval Jelly by Loctite brand. It is very thick, like pudding and I also have a supply of small inexpensive paint brushes, and tiny bristle brushes. The paint brushes were 5 in a pack for maybe $3.00 by Crayola and the bristle/bottle brushes were 5 together in various sizes from Ace hardware for maybe $5.

I would use a bristle brush " cleaner to clean inside the guides, then a Q-tip to dry it off. Then use a small paint brush to paint the Naval jelly inside the guide, wait 20 min. Then use Q-Tips to remove the crud. I get those 500 per box Q-Tip generic brands.

Oxalic acid would be my first choice. However, sometimes the rust is under the surface where the oxalic acid doesn't penetrate. Before I starting using oxalic acid, I came up with the following approach. First I wrapped some sandpaper around a heavy gauge wire. Strips of sandpaper could then be torn off to reveal fresh sandpaper as needed. I bent the wire a little so to avoid contacting the rest of the top tube and added extra tape around the guide for added protection.

What ever you do, do it carefully! Those darn guides are not always attached all that strongly. I have snapped a couple off when attempting to round out or clean out the little guys.

IF you can get a real good paint match,

and

IF the guide isn't salvagable

you COULD braze a new one on and touch up the paint.

I'd do EVERYTHING ELSE suggested here prior to doing that.

Naval Jelly works well for me. It sometimes takes multiple applications. I use acid brushes and I sneak Q-Tips from the bathroom.

The issue that usually comes up is that there's just not that much material that makes up those tiny cable guides, so if you are not careful, with taking off the rust with mechanical methods, what's left of it under all the rust might not even be enough to support the brake cable without eventually breaking up, especially the most forward one......
So if your bike starts rusting at the TT cable guides, it is critical you give it some attention asap, before it gets too bad to fix....