Rust underneath: where to look and what is already too late

Corrosion is the only “fault” that sets no codes, lights no lamps, and does not stop the car driving — until it becomes the reason the car is worth nothing.

Where it starts

Not where you can see it, but where moisture lingers longest.

Inside the sills — the main location. Water enters through the drains and stays if they are blocked.

Rear arches — a zone of constant blasting from the wheels.

Lower door edges — they have drain holes too, and those block too.

Suspension and subframe mountings — a critical zone, because these are structural.

Seam welds and points under the boot seal.

Under the cabin sound deadening — if there has been water inside; covered in the water leak article.

Under stickers, trims, and mouldings — anywhere moisture under plastic cannot dry.

Surface or perforating

The main distinction, and it decides the car’s fate.

Surface rust — a brown patch with sound, solid metal beneath. It cleans up and treats; the repair is cheap.

Blistering paint — there is already a bubble under it. The metal is thinner than it looks.

Perforation — a hole, often hidden under underseal or filler. That is a welding repair.

A simple test: tap it. A ringing sound means live metal; a dull one means a bubble or a layer of filler under the paint.

Do not be afraid to press a suspicious spot on a sill with a finger. If it gives, this is no longer cosmetic.

What to do in each case

Surface — clean back, treat with a converter, prime and paint. Do it now, because a season later it belongs to the next section.

Blistering — take it back to clean metal; touching up over the top is pointless.

Perforation in non-structural panels — welding and corrosion protection.

Perforation in structural members — chassis rail, suspension mounting, pillar. That is a safety matter, and here it is either a proper bodywork repair or the car is finished.

Prevention that works

Clear the drains — sills, doors, and the scuttle below the windscreen. The most effective action and it is free.

Wash the underside after a salted winter — not just the bodywork, specifically underneath.

Treat the cavities — the work that happens not on visible surfaces but inside sills and pillars.

Touch up stone chips immediately — a chip on the bonnet becomes a brown patch within one season.

Do not park a wet car in a warm closed garage without ventilation: warmth and moisture accelerate the process.

When buying used

Look underneath, always — with a torch and without hurrying.

Fresh underseal should raise suspicion: often it is concealment rather than care.

Check the pillars and suspension mountings — the parts that are not cosmetic.

A magnet helps find filler on body panels but does not work through underseal.

The general inspection protocol is in the used car article.

In short

• Corrosion starts inside sills and arches, not where you can see it.
• A dull sound when tapping means a bubble or filler.
• Blistering paint means the metal is already thinner than it looks.
• Perforation in structural members is a safety matter, not a cosmetic one.
• Clear drains are the most effective and cheapest prevention.

Download the app — codes across every system during a used car inspection are read with an ELM327 adapter over Bluetooth SPP on Android.