What paint thickness reading suggests filler?

Factory paint is usually 80–180 microns. Readings of 250–350 microns suggest a heavier repair or second respray, and anything over 350 microns likely means filler underneath - 500 microns or more is almost certainly filler rather than paint.

Is a repainted bumper a deal-breaker?

No. Bumpers take stone chips and car park scuffs, so a resprayed bumper on a car over five years old is completely normal and not a reason to walk away.

How do I spot overspray on a used car?

Run your fingers along rubber seals and check door shuts, under the bonnet and plastic trim. Overspray feels like fine sandpaper sitting on clean rubber.

Does fresh underseal mean hidden rust?

Not always, but fresh underseal sprayed over brake lines, sills or welding is often hiding corrosion. Check the online MoT history for past corrosion advisories first.

The Used Car Bodywork Check: How to Spot Accident Repairs Before You Buy

You can forgive a car a lot of things. A kerbed alloy, a shiny driver’s seat bolster, even a clutch that’s clearly on its way out. Those are known costs. Hidden accident damage is different, because the bit you can’t see is the bit that keeps you safe in the next shunt — and the bit that quietly guts your resale value three years down the line. A proper used car bodywork check takes about fifteen minutes. It is genuinely the best return on effort in the whole buying process.

Set yourself up properly

You don’t need much. A torch, a fridge magnet wrapped in a soft cloth, and a paint thickness gauge (£20–£35 online, and worth every penny) will do. A bottle of water helps too — pour it over a dry panel and the reflections sharpen up nicely.

Go in daylight. A bright overcast day is ideal: direct low sun flatters dents and hides colour mismatch, whereas flat light shows both. Start three or four metres back and look at the whole car. Then squat down and look along each flank, sighting down the side like you’re checking a snooker cue.

Panel gaps and shut lines: the fastest tell

Factory gaps are boringly consistent. Usually 3–5mm, tapering evenly from one end of a panel to the other. Run a finger along the gaps between bonnet and wings, doors and wings, doors and rear quarters, and boot or tailgate.

What to look for

A gap that’s visibly wider at one end than the other, or a door sitting proud or sunk at its trailing edge.
A bonnet that doesn’t sit flush with the wings, or a boot lid riding high.
One side of the car noticeably different from the other. Compare them directly.
A door or boot that needs a shove to close properly. That’s usually a bent hinge or a twisted shell.

Bolts and stickers

Look at the bolts holding the wings, bonnet hinges and door hinges. Spanner marks, chipped paint around the heads and missing plastic caps all hint that a panel has been off. Many manufacturers stick VIN stickers on wings, doors, bonnet and boot — their absence on one panel but not the others is a strong clue that panel is a replacement.

Overspray: the giveaway that never quite hides

Overspray is airborne paint landing where it shouldn’t. Check:

Rubber seals around windows, doors, boot and sunroof. They should feel smooth and clean, not like fine sandpaper.
Plastic wheel arch liners, under-bonnet sound deadening, the fuel filler surround and textured black trim.
Wiring and connectors under the bonnet — a fine grey dust over the loom is a classic tell.
Badges and chrome: masking lines, or badges refitted slightly crooked.
Inside the door shuts and under the sills, the hard-to-mask areas where a rushed job always slips.

A professional bodyshop masks carefully and leaves very little. A weekend job in a lock-up leaves it everywhere.

Paint thickness gauges: the cheapest forensic tool you’ll own

A gauge reads total film thickness in microns (µm) and works on steel and aluminium. It won’t read plastic bumpers, GRP or carbon, so ignore bumper readings entirely — they mean nothing.

Rough guide:

80–180 µm — normal factory paint. Modern cars are often 100–150 µm.
180–250 µm — likely a respray or a blend. Not necessarily bad.
250–350 µm — a heavier repair, or a second respray.
350 µm+ — probably filler underneath. Over 500 µm and you’re almost certainly measuring filler, not paint.

The consistency matters as much as the number. Take five or six readings across each panel. A jump from 120 µm at one end of a door to 400 µm in the middle means something happened there.

Filler, and the reflection test

A fridge magnet won’t stick to filler — but remember that plenty of modern bonnets, boots and wings are aluminium or plastic, where a magnet won’t stick anyway. Only use this on panels you know are steel, and put a cloth under the magnet so you don’t add a scratch of your own.

The knuckle tap is old but useful. Tap the panel gently: factory steel rings, filler sounds dead and dull.

Best of all is the reflection test. Stand so a straight line — a lamp post, a wall edge, a garage ceiling light — reflects along the panel. Waviness, a dip or a bulge shows up instantly against what should be a flat surface. Then look along the panel from a low angle with a torch: sanding scratches (pig-tails) under the clearcoat, dry spray patches and a different orange peel texture are all giveaways. Finally, check the base of the pillars, the wheel arch lips and inside the door shuts for a hard masking line where new paint stops.

Mismatched panels and colour shift

Silver, gold, red and pearlescent finishes are the hardest to match, and a slightly different shade after a repair is very common. Look at the car in shade and in sun, from four metres and side-on — colour shifts show most in shade. Note that plastic bumpers often look a shade off the body even from new, because they’re painted separately, so a mild difference proves nothing on its own. A door a shade off the wing, or a bonnet a shade off both wings, is worth a second look.

Fresh underseal: protection or cover-up?

This one matters most on older cars. Underseal is legitimate — plenty of owners have it done and it does slow corrosion. But if it’s wet-looking and glossy, sprayed over brake lines, fuel lines, the exhaust, suspension bolts and the bottoms of the sills, be suspicious. It’s a classic way to hide rot and welding.

Check the car’s MoT history online — it’s free. If it flagged sill or subframe corrosion two years ago and the underside is now gleaming black, ask some direct questions. Fresh underseal on a car with no history of it, stopping abruptly mid-floorpan, is rarely good news.

What’s normal for age — and what’s a deal-breaker

Perfectly normal

A resprayed or replaced bumper on anything over five years old.
Stone chips on the bonnet leading edge and front bumper.
A car park ding repaired on a door.
One panel reading 200–250 µm on a ten-year-old car, provided the shut lines around it are intact.
A slightly different shade on a plastic bumper.
Older paint that’s tidy but not factory-perfect.

Time to walk away

Panel gaps visibly uneven down one side, or a door or boot that won’t close cleanly.
Creases or ripples in inner wings, suspension turrets, boot floor or chassis legs. This is structure, and structure is the line.
Filler you can detect over a large area, especially around a wheel arch or sill.
Airbag warning light on, or a steering wheel or seat clearly newer than the rest of the interior — a sign of a heavy frontal impact.
Overspray everywhere plus masking lines in the door shuts. Someone did this badly.
Fresh underseal over visible welding near suspension mounts.
A car that shows on an HPI or history check as Cat S (structural write-off) or Cat N (non-structural) while being sold as never repaired. Cat S cars typically fetch 25–40% less than a clean equivalent and Cat N around 10–20% less. If you didn’t know, you’ve overpaid.

Your 15-minute pre-purchase bodywork checklist

View from four metres, then squat and sight along each flank.
Check every panel gap with a finger and compare left with right.
Inspect wing, bonnet, door and boot bolts for spanner marks and missing VIN stickers.
Run a hand along every rubber seal looking for overspray.
Check under the bonnet, inside the boot and in the door shuts for masking lines.
Take five or six paint gauge readings per panel and write the numbers down.
Magnet test on known steel panels, cloth underneath.
Reflection test along both flanks for waviness.
Get under the car with a torch — brake lines, sills, subframes.
Check the free MoT history, then pay for an HPI or vehicle history check.
If anything doesn’t add up, spend £100–£250 on a professional pre-purchase inspection. It’s cheaper than a surprise.

The bottom line

A tidy car and a straight car are two different things, and sellers know it. Cosmetic repairs are fine — expected, even, on anything with a few birthdays behind it. What you’re hunting for is dishonesty: fresh paint hiding a twisted shell, gloss underseal hiding rot, a Cat S car sold as a minter. Judge repairs on what they’re covering, not on the fact they exist.

When the numbers stack up and the panels are straight, that’s the moment to move. UpCars.uk is a good place to look when you’re ready, since the cars listed there are checked over properly before they go up — so you can spend your fifteen minutes confirming what you’ve already been told, rather than hunting for what you haven’t.