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Cars and vehicles

Forty microns of clear coat doing all the work, and a wheel finish whose own standards body forbids the products sold to clean it.

What this area turns on

Almost everything that looks like a stain on car paint is an etch in the clear coat: bird droppings, tree sap, insect residue and egg are acidic or alkaline enough to dish into it, especially in sun, so washing removes the deposit and leaves the mark. And the standards guide for finished aluminium — which is what an alloy wheel is — names trisodium phosphate, phosphoric acid, hydrochloric acid, hydrofluoric acid and fluorides as not to be used on anodised surfaces, with damage that may be permanent. Those are most of the acids in a wheel cleaner.

Assessed and found wanting

Widely repeated techniques in this area that the evidence does not support, or supports only in part.

  • Acidic wheel cleaner on alloy wheels

    Disputed

    Sold specifically for the job, and a standards body prohibits every acid it is likely to contain on the finish it is likely to be sprayed on.

The records

  • Car exterior

    Thing to clean

    Almost everything that marks car paint is an etch in a clear coat forty microns thick, and almost every scratch comes from grit under a cloth.

  • Vehicle clear coat

    Material or surface

    A transparent film a fraction of a millimetre thick doing all the work. Everything that looks like a stain on car paint is usually an etch in this layer.

  • Alloy wheel

    Component

    A coated aluminium casting collecting hot iron dust from the brakes. Two of the finishes it comes in are damaged by the acids sold to clean it.

  • Windscreen

    Component

    Laminated glass with a wiper dragging across it. Most windscreen smearing is a film the wiper is spreading rather than a wiper that has worn out.

  • Anodised aluminium

    Material or surface

    An oxide layer grown into the metal rather than laid on it. A standards body names five specific chemicals that must never touch it, and the damage is permanent.

  • Powder-coated aluminium

    Material or surface

    A baked organic coating over metal. Strong solvents and abrasives attack the coating, and the rule of thumb for what is mild enough is unusually memorable.

  • Tree sap

    Stain or problem

    A resin that hardens on exposure and bonds tightly to what it lands on. Left on paintwork in sun, it etches rather than merely sticking.

  • Egg stain

    Stain or problem

    Almost pure protein, and the clearest case in the catalogue where hot water makes the problem permanent rather than smaller.

  • Road salt

    Stain or problem

    A white tidemark on boots, carpet and car sills that is also actively corroding whatever it is sitting on.

  • Chloride attack

    Science

    The specific reason stainless steel does rust. Chloride ions break the passive film locally, and the damage concentrates in a pinpoint.

  • Acidic wheel cleaner on alloy wheels

    Claim or technique

    Sold specifically for the job, and a standards body prohibits every acid it is likely to contain on the finish it is likely to be sprayed on.

  • Neutral cleaning

    Method

    The default that four separate trades converge on independently: a neutral detergent, warm water, a soft tool and a thorough rinse.

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