How do I get rid of rust that keeps coming back on a chrome fitting?

Why the usual answer works

Acidic and chelating rust treatments dissolve or bind iron oxide, and a fine abrasive lifts it mechanically. On rust sitting on top of intact plating, both genuinely work and the fitting comes up bright.

Usually recommended: Mild cream cleanser.

Why it must not be used here

Where the plating has been breached, the rust is not sitting on the surface — it is the steel underneath corroding and showing through. Removing what is visible does nothing to the corrosion producing it, and the treatments make the situation worse in two ways: an acid or chelant reaching bare substrate through a breach attacks it directly, and abrasion widens the breach that let moisture in. The mark returns, faster each time.

This damage cannot be undone by cleaning. There is nothing left on the surface to remove.

More about chrome plating and rust.

What to do instead

Establish which situation you are in before treating anything, because the two look similar and need opposite responses. Surface rust transferred onto intact plating — from a tin, a razor, or steel wool used nearby — is a cleaning problem and a mild non-scratching cream is the trade-recommended treatment. Rust emerging from beneath plating that is pitted, lifting or scratched through is corrosion of the base metal, and no cleaning method addresses it; the fitting needs re-plating or replacing. The tell is behaviour rather than appearance: if it returns in the same place within weeks of being removed, it is coming from underneath. Note also that rust spots on stainless steel are usually neither of these — they are particles of ordinary carbon steel shed by wire wool, corroding on a surface that is itself intact.

• Mild cream cleanser — A liquid carrying a fine soft abrasive. The trade answer for water marks and light staining on stainless steel — and not the same thing as scouring powder.
• Rinsing and drying — The step people skip, and the one that decides whether cleaning protected the surface or damaged it. Residue left behind is exposure that continues after you stop.

This is a case where a specialist is a reasonable answer. Restoring a surface is a different trade from cleaning it, and continuing to try cleaning products on damage generally makes the eventual restoration harder.

The chemistry behind both halves

• Corrosion and passivation — Most corrosion-resistant metals are protected by an invisible oxide film. Cleaning either preserves that film or breaks it, and breaking it is how stainless steel rusts.
• Material compatibility — The reason a useful answer is about the surface rather than about the product. The same agent is the right answer and the wrong answer depending on what it lands on.