The engine bay brake and fuel / vapor lines are galvanized steel right ? I'm thinking of treating them with Ospho phosphoric acid and then painting them with a small brush. I think I read somewhere you're not supposed to ospho galvanized steel but maybe that only applies to new or un-rusted  parts.

I coated my fuel lines under the battery with SEM Rust Inhibitor.  They've held up for the last two years, when I did them.  Turned them a dark almost black color.  Never recoated them with any type paint. 

No answers for your questions just my experience on my car. 8^)

I saw the SEM, it's phosphoric acid. It's what my Dad recommended some years ago. I think the lines are more discolored than rusty, hope so anyway. There's also zinc-rich paint. I've never used it, but the building dept. says to use it on welds exposed to the weather after inspecton. But in that case it's going on fresh weld metal, not old discolored galvanized brake lines.

Edited December 27, 20178 yr by Stanley

Zinc rich paint works great on old discolored galvanized metal.

I used 0000 steel wool to clean mine, only to see how pitted they were.  I ended up cutting a short section out and replacing with fuel hose.  It is the supply line straight underneath the battery.  The return side wasn't that bad so I left it, coated with the SEM product.  One day I'll replace them with new lines, right after I replace the old poly sheeting under my house.