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I have a growing oxidation spot on the bottom of my BB shell. I think the frame is made out of 7005 aluminum (it’s a 2004 Colnago Dream). The cable guide is attached using a steel screw (why don’t they use plastic?), and the oxidation originated at the screw hole, so the oxidation is obviously a result of the contact by the two dissimilar metals, plus water, sweat, etc. I’m going to try to find a plastic screw that can hold the cable guide in place, but I’d also like to stop the “cancer” from growing, and I’m hoping for guidance.

My plan is to (i) disassemble the BB area – remove cranks and BB so that I can access both the inside & outside of the BB shell, (ii) remove the oxidation with a wire brush and a bit of sandpaper both inside and out, so that I’m down to nearly clean aluminum, (iii) then paint the spot, probably using primer then some silver paint over that (to roughly match the original silver paint in that area). I’m not picky about the cosmetics, so I don’t need a perfect match with the original paint. I believe the Colnago paint is water based.

Has anyone done this before? Anything I’m missing? Any thoughts or suggestions?
 

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Small area, but important.

Make sure you run a small brush loaded with primer, then paint, around the inside of the small screw hole after you cleaned it.

Many times, this is exactly where corrosion starts - as you guessed.Both surfaces of a piece of metal might be coated nicely with primer and paint, but then a small hole gets drilled through the metal. Unless the bare metal on the inside of that small hole is painted, it becomes the starting point for corrosion.

A notorious example were late 80's Cannondales. After the painting, six small holes were drilled into the top tube to accept the snap pins of three plastic cable guides. Corrosion and bubbling paint around those cable guide holes was legendary.
 
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