Prolonging the Life of Poorly Accessible Rusted Steel

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I had a rear 3/16" OD steel brake line in my car fail due to rust. I spliced out the area with the hole with copper nickel line, but, I noticed that there's areas closer to the engine where the painted surfaces have bubbled a bit. Unfortunately, access is such that I can't splice fresh line, but I can get a hand up there, and, to a point, maybe sand down the surface and paint it. Due to the gauge of the tubing, though, I can't sand that aggressively, so, no matter what, I can't remove every bit of rust.

I have a very old car that I don't plan on keeping more than a few months. I'm just trying to find an approach that buys me the most time. Right now, I'm thinking maybe tannic acid and then rustoleum. Or maybe zinc paint? Thoughts?

Accessing the lines involves both a window of good weather and a pretty hefty chunk of time lifting the car high enough and removing the protective shield, so whatever I come up with needs to be a one shot deal. Again, not looking for forever, just a few months. There's a good chance, that, doing nothing, I'll be fine for a few months, but, I'd still like to hedge my bets by trying something.

And, just to be clear, there's no need for a safety lecture. When my rear brake line failed, the car still stopped fine. If this were a front brake, I'd be a lot more cautious about slightly rusty lines, but, it's not. My front brakes are flawless.
 
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Rust may be working its way out, since brake fluid is highly hygroscopic.
Also, when one branch fails, most cars lose pressure in one front brake and the opposite rear brake (rather than both front or both rear calipers).
There are commercial products that stop the rust without the need for over-painting the affected surface.
 
Lnewqban said:
Rust may be working its way out, since brake fluid is highly hygroscopic.
Also, when one branch fails, most cars lose pressure in one front brake and the opposite rear brake (rather than both front or both rear calipers).
There are commercial products that stop the rust without the need for over-painting the affected surface.

Interesting. Regarding the hygroscopicity. So, potential water in the previously aging fluid might have been contributory. Assuming the corrosion was internal, now that the fluid has been aggressively replaced with fresh fluid, that might slow the corrosion down, correct?

What commercial products are you referring to?
 
Slightly off-topic for your brake lines, but here goes:

Generally, any acid will remove rust. Check the label on some of them, many are phosphoric acid which is a decent medium-strength rust remover.

Don't use strong acids like hydrochloric (muriatic) or sulfuric acid, they strip off so much of the metal surface that the rust returns with a vengeance.

For gentler action, there is lemon juice or vinegar.

You also have a choice of liquids or gels. Gels are great for larger items that you don't want to dunk in a large container. For smaller and/or more detailed objects I lean towards immersing in the liquid formulations; or sometimes saturating a rag and just wiping the surface.

Cheers,
Tom