Stone chips and scratches: repair before it costs more

A windscreen is laminated glass: two layers of glass with a plastic interlayer between them. A stone chip hits the outer layer and leaves a small crater with fine cracks around it. As long as it stays that way it is a blemish. But the glass works: the body flexes over every bump, the sun heats the screen, the heater warms it from inside on a cold morning. Any of those stresses can turn the crater into a crack, and a crack is no longer repaired.

With paint it is a different mechanism with the same outcome. Paint protects the metal, and where it is missing, steel starts to rust. The rust does not stay where the chip is but creeps under the intact paint, lifts it, and turns a dot into a patch. Salt and moisture speed that up. A scratch that only reaches the clear coat, by contrast, is a cosmetic problem; one that goes down to bare metal is a chip in the shape of a line.

That is why autumn is the right time for both: the glass still has the frost ahead of it, the paint the salt, and it is still warm and dry enough for resin and paint.

In a repair the crater is cleaned, filled with a clear resin under vacuum, and the resin is cured with UV light. That stops the damage growing and largely restores strength. It does not become invisible; a small spot usually stays visible. The job is quick and the screen stays in the car.

Whether it can be repaired depends on three things. Size: the industry and manufacturers set a limit beyond which a repair will not hold. Position: in the field of view directly in front of the driver a repair leaves a distortion, so as a rule the screen is replaced there, and near the edge the glass is under more stress and cracks more easily. And age: a chip that has taken in dirt and water for weeks is harder to fill. Until the appointment, a piece of clear tape keeps the spot clean and dry.

On newer cars there is also the area behind the rear-view mirror. The camera for lane-keeping and emergency braking sits there, and damage in its field of view is generally not repaired, because any distortion disturbs its measurements. After a replacement the camera has to be calibrated. When booking, ask about that explicitly, and about the screen variant: acoustic, heated, head-up display — a screen that only looks similar does not necessarily fit.

Whether damage in the field of view counts as a defect at the periodic test, and whether you may drive with it, is up to each country; if in doubt, the testing station can tell you.

In many countries glass damage is part of partial comprehensive cover or a separate glass cover, but not in every policy and not always for all glass. Before booking, look at your own terms: whether glass is covered at all, whether only the windscreen or also side windows, rear screen and glass roof, how big the excess is and whether the insurer prescribes a particular workshop.

Many insurers treat repair and replacement differently, because the repair is much cheaper for them — with some, the excess falls away entirely for a repair. That is another reason to act early: wait until the crack appears and you pay the excess on a replacement. Whether a glass claim affects your no-claims discount depends on the cover; ask rather than assume.

Two rules of caution. If someone approaches you at a filling station or in a car park offering a free windscreen replacement, sign nothing before you have spoken to your insurer — least of all an assignment of your claim to the business. And holding someone responsible for a stone chip almost never works: it is rarely possible to establish who threw up the stone. Scratches and paint damage fall under quite different covers depending on the cause, a parking knock differently from deliberate damage; what applies is in the policy.

Stone chips in the paint collect on the leading edge of the bonnet, the front bumper, the door mirrors, the sills behind the front wheels and the rear wheel arches. Wash the car thoroughly once in autumn, search it in good light, and you will find nearly all of them. On plastic parts such as the bumper a chip is cosmetic; on metal what matters is whether bare metal is already showing.

Where bare metal shows, it gets sealed, in the right order. The spot is cleaned and degreased; any starting rust is removed with a glass-fibre pen or a fine tool until bright metal shows. Then a thin primer, on top of it the touch-up paint in the car's colour — depending on the make, the paint code is in the door frame, under the bonnet or in the boot — and on most finishes a clear coat last. Several thin dabs with drying time between them rather than one thick drop.

The result is visible up close and not from two metres away, and that is not the point anyway: the point is that no water and no salt reach the metal. Touch-up paint needs warmth and dry air to cure, which is why this job belongs in autumn rather than January. The same applies to aluminium panels, even though the corrosion there is white rather than brown. With many chips on the bonnet, a clear protective film applied after the touch-up keeps the next ones off.

Paint is built up in layers: under the clear coat lies the base coat carrying the colour, below that filler and primer, then the metal or plastic. How deep a scratch goes decides the repair, and a simple test gives a first hint: run your fingernail across it, and if it does not catch, the scratch is usually only in the clear coat.

Scratches like that can be polished out. Polishing removes a thin layer of clear coat until the scratch disappears — that cannot be repeated indefinitely, and with a machine it belongs in practised hands, because it is easy to go through on edges. If your nail catches, polishing is not enough. For a narrow scratch, touching up as with a chip helps; the result protects, but it stays visible.

In between lies smart repair: a workshop repairs a small area without painting the whole panel and blends the new paint into its surroundings. It suits individual scratches, scuffs on the bumper or kerbed alloy wheels; dents without paint damage can often be pushed out without any paint at all. The method has limits on large areas, on edges and with effect finishes such as metallic or pearl, whose shade is hard to match over a small patch. Then painting the whole panel is the more honest solution.

Before a sale the question changes: not what helps the car, but what a buyer sees and how they read it. A repaired stone chip in the windscreen is almost always worth it. It is cheap, often covered by insurance, and a buyer who spots a crack or a crater in the field of view deducts a new screen with calibration from the price — and wonders what else has been left.

Touched-up chips and polished-out scratches are worth it too, because they show care. With repainting whole panels shortly before a sale it is the other way round: it costs a lot, and a buyer with a paint thickness gauge finds it in seconds and asks about an accident. If you do have paint work done, keep the invoice and show it; that turns a suspicion into information.

Whatever you do not repair, you mention. Good daylight photos show chips and scratches as they are, and the listing says where they are. On Carvexia the seller's statements include a question about known defects for this; the answer appears on the vehicle page as a seller's statement, not checked. A buyer who finds a scratch you announced negotiates about the scratch. One who discovers it themselves negotiates about everything.