Car exterior
Almost everything that marks car paint is an etch in a thin clear coat, and almost every scratch comes from grit under a cloth.
Start here
What this is, what tends to go wrong with it, and what to do about it.
Made from
Vehicle clear coat, Glass, Natural rubber, Household plastics
Which part are you cleaning?
These parts take different answers. Cleaning one the way you clean another is a common way to damage this.
- Alloy wheel
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.
Powder-coated aluminium, Aluminium, Lacquered metal
Things not to use here
- Windscreen
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.
Glass
Things not to use here
- Chrome and brightwork trim
A plated layer that every polish makes thinner. Clean it gently once, then coat it so it needs polishing less often.
Chrome plating, Copper and brass
Things not to use here
What tends to go wrong
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.
Why it is difficult: Solubility and polarity
The one stain where the first move is mechanical and the chemistry barely matters — and where the folk advice to let it dry first is not what the trade body says.
Mostly textile fibres and skin, held by static as much as by gravity — which is why a dry cloth on a plastic surface makes it worse.
Why it is difficult: Static and dust, Mechanical action and abrasion, Why different surfaces get different dirt, Surface roughness
A seasonal particulate that is part of ordinary dust for most of the year and overwhelms it for a few weeks. Dry removal first, because wet pollen stains.
Why it is difficult: Why different surfaces get different dirt
- Mouldvia its material
A living growth, not a stain. Killing it and removing the staining are two separate jobs, and neither addresses the moisture that caused it.
Why it is difficult: Porous and non-porous surfaces, Moisture control and recurrence, Cleaning, sanitising and disinfecting, Organic load, Surface roughness
- Limescalevia its material
A hard, chalky calcium carbonate deposit left by hard water. Dissolved by acids, essentially untouched by detergents.
Why it is difficult: Water hardness, Crystallisation, Adhesion and cohesion
- Rustvia its component
Hydrated iron oxide from corroding iron or steel. Distinguish a surface deposit, which cleans off, from active corrosion of the item itself, which does not.
Why it is difficult: Corrosion and passivation, Why old stains are harder, Adhesion and cohesion
- Burnt-on food and carbon depositsvia its component
Food heated until it carbonised and bonded to the surface. Time and softening do the work; scouring powder does the damage.
Why it is difficult: Contact time, Carbonisation, Adhesion and cohesion, Why old stains are harder
- Water marksvia its component
What hard water leaves behind when it evaporates instead of being wiped away. A mineral film, not dirt, and the fix is at the drying stage.
Why it is difficult: Water hardness, Evaporative deposition
- Greasevia its component
Fats and oils, which water alone cannot lift. Removed by surfactants, by heat, by solvents, or by alkalis that convert the fat into soap.
Why it is difficult: Why oil resists water, Emulsification
- Road salt· no established method
A white tidemark on boots, carpet and car sills that is also actively corroding whatever it is sitting on.
Why it is difficult: Chloride attack, Evaporative deposition
- Egg stain· no established method
Almost pure protein, and the clearest case there is of hot water making the problem permanent rather than smaller.
Why it is difficult: Protein denaturation, Heat-set stains
- Bird droppings· no established method
Acidic, alkaline in places, and gritty. On vehicle paint in sun it etches within hours, and what remains after washing is a mark in the finish rather than on it.
Why it is difficult: Why old stains are harder, Coating thickness
- Insect residue· no established method
Protein, and therefore a stain that heat sets. A summer bonnet is a protein deposit baked onto a clear coat by the engine and the sun.
Why it is difficult: Heat-set stains
- Brake dust· no established method
Hot iron particles embedded into a wheel coating, not dust sitting on it. They corrode where they land, which is why an alloy wheel appears to be rusting.
Why it is difficult: Adhesion and cohesion, Coating thickness
Pressure-sensitive adhesive left behind when the paper tears away. It stays tacky by design, which is why it collects dirt and turns grey.
Why it is difficult: Adhesion and cohesion, Surface roughness
A concentrated water-soluble dye with nothing else in it. Nothing has to be emulsified or dissolved — the only question is whether the surface has taken the colour up.
Why it is difficult: Capillary absorption, Dye migration and colour bleeding
A sticky, yellow-brown tar film deposited everywhere at once. It is the reason a room smells after everything visible has been cleaned.
Why it is difficult: Adsorption of odour molecules, Solubility and polarity
Coloured by turmeric in most prepared mustards, and curcumin is a pH indicator — which is why an alkaline cleaner turns a yellow mark red.
Why it is difficult: pH indicators in stains, Fat-soluble pigments, Photodegradation, Pigment retention
The commonest cleaning problem that is not dirt. The tackiness is the plastic's own additive arriving at the surface, which is why wiping it off works for a week.
Why it is difficult: Plasticizer migration, Damage versus soiling
The science behind this: Etching versus staining, Gloss and light scattering, Mechanical action and abrasion, Coating thickness, Polishing removes metal, Leaching and clouded glass
In detail
Vehicle paint is a coloured base coat under a transparent clear coat that carries the gloss and all of the chemical resistance. Two mechanisms account for most damage. The first is etching: bird droppings, tree sap, insect residue and egg are acidic or alkaline enough to dish into the clear coat, particularly in sun, so washing removes the deposit and leaves a mark that needs polishing rather than cleaning. The second is abrasion, and it is entirely avoidable. The glass industry's account of scratching generalises exactly — the cause is abrasive particles trapped between the surface and the tool rather than the cleaner, and enough water to float them away is the preventive. That is the argument for rinsing thoroughly before a cloth touches the paint, for two buckets, and for never wiping a dry, dusty panel. Two standing instructions come from the metal-finish guidance: do not clean a hot, sun-heated surface, and identify the finish first, since a wrapped or matt panel tolerates none of the polishing a gloss clear coat does. The finish and glass rules here come from architectural guidance, applied to a car by analogy; no vehicle maker's own exterior care instruction is drawn on.
Related
- Car exterior has the component Alloy wheel
- Car exterior has the component Windscreen
- Car exterior has the component Chrome and brightwork trim
- Car exterior is made from Vehicle clear coat
- Car exterior is made from Glass
- Car exterior is made from Natural rubber
- Car exterior is made from Household plastics
- Car exterior is commonly affected by Tree sap
- Car exterior is commonly affected by Road salt
- Car exterior is commonly affected by Egg stain
- Car exterior is commonly affected by Mud
- Car exterior is commonly affected by Dust
- Car exterior is commonly affected by Bird droppings
- Car exterior is commonly affected by Insect residue
- Car exterior is commonly affected by Pollen
- Car exterior is commonly affected by Brake dust
Sources for this page
Sources
Glass Technical Paper FB01-00: Proper Procedures for Cleaning Architectural Glass Products
National Glass Association (NGA) with the International Window Cleaning Association (IWCA), 2023 · Trade or professional body
Relied on for: That scratching is caused by abrasive particles trapped between the surface and the tool rather than by the cleaner, and that sufficient water helps prevent it. (inferred from the source; not stated directly)
AAMA 609 & 610-02: Cleaning and Maintenance Guide for Architecturally Finished Aluminum
American Architectural Manufacturers Association (AAMA), 2002 · Standards body
Relied on for: That hot, sun-heated surfaces must not be cleaned because reactions are accelerated and non-uniform, and that the finish must be identified before a method is chosen. (inferred from the source; not stated directly)