Cap With a Large Yellow Stain: Before and After

This cap looked scary, but it was really just a single stain without many complicating factors.

One thing to watch out for: some shops soak caps using a redox (oxidation-reduction) method, but that can speed up oxidation and rust on the top button and the adjustable buckle at the back.

Once rust sets in, unless the part is replaced, you can only remove what's visible on the surface.

Here's how it looked after cleaning:

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