Cosmetic repairs are normal on used cars: a resprayed bumper or a replaced wing says little about the car. Structural repairs are different, because they change how the car behaves in the next collision. The skill is telling the two apart, and it can be done in thirty minutes with a paint thickness gauge, a torch and a methodical walk-round.
Cosmetic panels and the structure
| Bolted on: repair is usually cosmetic | Welded in: repair is structural |
|---|---|
| Front and rear bumpers | Front rails (the longitudinal members behind the bumper) |
| Front wings on most cars | Strut towers and inner wings |
| Bonnet and boot lid | A, B and C pillars |
| Doors | Sills and the floor pan |
| Headlamps, grille, mirrors | Roof and rear quarter panels |
| On some cars, the radiator support | Rear rails and the boot floor |
A replaced bolt-on panel, correctly fitted and painted, is a price point. Evidence of cutting, straightening or welding in the right-hand column is a different category of car.
Reading paint thickness
A paint thickness gauge measures the coating on a steel panel in micrometres. Factory finishes on steel commonly read in a band of roughly 80 to 160 micrometres, and each manufacturer's finish is consistent across the car. That consistency is what you are testing:
- Take several readings on every panel: the centre and each corner.
- Compare left with right and each panel with its neighbours, rather than with an absolute number.
- A panel reading two to three times its neighbours has been repainted. Readings of several hundred micrometres or more suggest body filler beneath.
- Aluminium panels need a gauge with a non-ferrous probe, and plastic bumpers cannot be read with a standard gauge at all.
Repainting alone is not a problem; it tells you where to look harder. A repainted rear quarter panel, which is welded to the structure on most cars, deserves a close look at the boot floor and the door aperture behind it.
Gaps, alignment and finish
- Panel gaps should be even along their length and symmetrical left to right. Compare the bonnet edges, the door gaps and the boot lid.
- Doors should close with the same effort and sound on both sides.
- Look along each side in low light for ripples, and for differences in the texture of the paint surface between panels.
- Check rubber seals, badges, lamps and plastic trim for overspray, and look for masking lines at panel edges.
- Check colour match in daylight from several angles; metallic paints show a mismatch most clearly at an angle.
Fasteners, welds and sealer
- Bolt heads. On many cars, the bolts holding wings, doors, bonnet and boot lid are painted in the factory with the body. Cracked paint or spanner marks on them mean the panel has been removed.
- Spot welds. Factory spot welds are uniform, round and evenly spaced, visible in door apertures and under the bonnet. Repair welds are usually irregular, larger and often ground.
- Seam sealer. Factory sealer is applied by machine in a uniform bead. Sealer that is uneven, brushed or smeared, especially in the boot and engine bay, points to repair.
- Underbody coating. Fresh underseal on part of an older car can be protection, or it can be concealment. Look at where it starts and stops.
The structural checkpoints
- Under the bonnet: front rails, strut towers and inner wings, for kinks, rippling, clamp marks from straightening equipment and non-factory welds.
- Door apertures: pull back the edge of the door seals to see the pillar flanges and their spot welds.
- Underneath: sills and jacking points, floor pan and rear rails.
- The boot: lift the floor covering and check the spare wheel well and the rear panel for creases, welds and filler.
- The roof: ripples or repair marks at the pillar joints suggest a rollover.
Glass, airbags and seat belts
- Every piece of glass carries a manufacturer mark and a date code. A replaced windscreen is common after stone damage; replaced side or rear glass is less innocent and suggests an impact or a break-in.
- The airbag warning lamp should light at start and go out. Check that airbag covers on the steering wheel and dashboard fit perfectly and match the surrounding texture and colour.
- Seat belts should retract smoothly and lock on a sharp pull. After a serious impact, pretensioners fire and must be replaced.
The test drive
On a straight, level road the car should track straight with the steering wheel centred. Uneven wear on the inside or outside edge of the tyres, a car that pulls, or knocking over bumps can all point to suspension or structural damage that alignment alone cannot correct. Listen for wind noise from doors that no longer seal properly.
What to decide
| Finding | Usual decision |
|---|---|
| Repainted or replaced bolt-on panels, well done, consistent gaps | Acceptable; reflect it in the price |
| Poor-quality cosmetic repair | Price for rectification, or walk away |
| Evidence of straightening or welding on rails, pillars, sills or strut towers | Walk away unless you have the repair documentation and an independent structural assessment |
| Airbag deployment without documented, correct repair | Walk away |
Ask for insurance claim and repair history where the country of origin makes it available, after confirming the VIN (how to decode it). Accident history often comes with other hidden history, so run the mileage checks and the flood checks at the same inspection.
New cars are not automatically exempt. Vehicles damaged in transport are sometimes repaired before sale. On a new car the paint readings should be uniform across every panel, and any exception deserves a written explanation; the arrival inspection method is in pre-delivery inspection for imported vehicles.