Have noticed some small areas of surface rust on fuel lines. Can/should I treat with POR-15 or similar product? Keep an eye on it? Or just replace?

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14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 159k miles
06 E320 CDI "Rutherford", Black on Tan, 178k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 145k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 75k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife

I just take some heavy grease and apply with a one inch paint brush. Even though you do not get complete coverage the oil in it creeps. It reduces the continuing rate of rusting quite substantially. At least in my experience.

Eventually it also picks up road dust. In fact a drive down a dusty unpaved road after application can stiffen it up even more. Do all of the lines though just not the areas that show rust. The oil creeps into any line mounting components where moisture gets in.

Light surface rust is different than heavy well advanced rust. How it works is simple. It blocks oxygen getting at the metal. Or slows the process down so much. You usually can just about forget those lines.

You can steel wool or scotchbrite pad off the loose rust and get out your neoprene gloves and a section of old T shirt and spray brake cleaner into the T shirt material and degrease the lines.
After that you can use grease or oil on the lines or you could go to Autozone or a hardware store and get some of that rust converter paint and paint the hard lines.

The rust converter paint deteriorates in sun light but that should not be an issue under the hood.

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84 300D, 82 Volvo 244Gl Diesel

The problem areas are often under the clips where you can't see the corrosion. Brake lines are subject to same corrosion.

If line looks OK except for some surface rust, it may be hard to paint in a way that gives complete coverage. What I would do, is spray the lines with a rust treatment. Here in Canada, many of us have the cars rust treated annually, so those lines get sprayed anyway. But the same products are available in spray cans. Names here are Rust-Chek and Krown. You probably can't get those? If not, Fluid-Film should do a good job. I have used that inside rockers and other rust prone areas. https://www.fluid-film.com/

This is a lot easier than painting - just have to do it more often!

When I replaced fuel and brake lines on my old 300D, I did paint them while new with DOM-16 (similar to POR15)

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Graham
85 300D,72 350SL, 98 E320, Outback 2.5

I literally just covered my engine bay in Loctite rust converter. It will work well after you 3M Pro Scotchpad all surface rust. Just be sure to let it dry for a few hours before the 2nd coat.

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Why I will never do business with "DieselKraut" again

http://www.peachparts.com/shopforum/mercedes-benz-used-parts-sale-wanted/378935-why-i-will-never-do-business-dieselkraut-again.html

Were you speaking of the Fuel Lines under the Car?

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84 300D, 82 Volvo 244Gl Diesel

Sorry for the ambiguity...no, fuel lines to injectors.

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14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 159k miles
06 E320 CDI "Rutherford", Black on Tan, 178k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 145k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 75k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife

PB-Blaster or its ilk and a bit of a scrub with a Scotch-Brite type of pad should do it.

I wouldn't use a rust converter on the lines. They were originally cadmium coated and I don't know how the remnants of the old plating would react to the chemical.

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“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now

That is what I thought you meant. That is what my answer was about.

However, I had the Fuel Lines under the Car rust through under one of the Clamps so that can also happen. Rust restoring paint would work OK there to prevent that and being sure the rubber isolators under the clamp are in good condition..

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84 300D, 82 Volvo 244Gl Diesel

I have 2 sets of fuel injection Hard lines and neither show any signs of ever having been plated.

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84 300D, 82 Volvo 244Gl Diesel

They were originally.

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“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now

Sorry, thought fuel lines under car. As for the actual injector lines. They are very thick walled. Esthetically you may want to clean them up but I doubt they would ever rust through. I still would apply some grease to the three lines under the car and the extension lines to the rear calipers. There was an old saying. It was less that a joke to many. You could lay in bed and hear your car rusting away in the severe rust belt area.

Wow, everything in the picture has no rust on it.

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84 300D, 82 Volvo 244Gl Diesel