The part was polished, inspected and put away. Later the same face is flat and grey: it reflects light as a haze instead of an image, and no amount of wiping brings the shine back. Often the edges are worse than the middle, or conspicuously better, depending on how the part was wrapped.
Dullness is a description, not a cause. A polished surface is a surface made of extremely fine scratches that all run one way. Anything that roughens it at a smaller scale, coats it, or etches it will destroy the reflected image, and each of those does a different amount of damage.
Storage is the one time a finished part sits in air that does not move, with no heat to dry it. Wrapped in plastic or bubble film, it holds a layer of humid air against the metal, and any temperature swing condenses water inside the film, where it cannot evaporate.
Cardboard and some papers are worse than neutral: they hold moisture and can be acidic. A part lying on a concrete floor in a cold store draws damp from the slab. None of this touches the bulk of the metal. All of it changes the surface, which is the only part anyone sees.
Polishing compound left sitting in the scratch pattern, protective film adhesive that has cured, and fingerprints all produce a dull patch without attacking the metal. The signature is a boundary: a dull area exactly under where the film was, with a clean strip at the edge the film never reached.
Fingerprints are the small version of the same problem. Salts and oils from skin hold moisture in a ring, and left long enough that ring can etch a faint mark which no longer wipes away, because it is no longer on the surface.
Three appearances get called dull. Etch is uniform and slightly rough, and it is worse wherever the wrap was tightest against the metal. Haze from a cloth that carried grit is directional, with fine lines along the direction of the wipe. Residue looks dull from one angle and disappears from another.
An etch will not clean off, and a residue will. That single difference sorts most of these cases before you touch the surface with anything abrasive, and it decides whether the answer is a solvent or a polish.
Wipe a small area with a soft cloth and a mild solvent, let it dry, and look again under a raking light. If the image comes back, you had a deposit and the metal was never damaged. If it does not come back, the surface itself has changed and cleaning will not recover it.
Then feel it. A fingertip, or better a fingernail, drawn across the patch and then across a good area tells you whether there is depth. Use a magnifier on the boundary: an etch has a soft edge, a scratch has a hard one, and pitting has visible pits at the centre of the dull patch.
Store finished parts dry, off the floor, in air that can move, and unwrap anything where the film is holding condensation. Use acid-free paper or a breathable sleeve rather than tight plastic for a part that will sit for a season. Handle polished faces with clean gloves rather than a rag that has been everywhere.
And accept that a polish is a service finish rather than a permanent one. It is maintained by cleaning and light re-polishing, which is why a titanium fabrication component is often ordered brushed where it will be handled and polished only where it will be seen. A titanium exhaust tip lives with exactly the same rule at the end of the car.
