Rust: where it shows up first and where it decides

Surface rust is a thin brown layer on bare steel, forming as soon as the paint or coating is broken. On the underbody, on bolts, on brake discs and on suspension parts it is normal and says nothing about the car’s substance. A car with an evenly brown underside is not automatically a problem.

The second kind is perforation: the steel loses section, the material flakes away in layers, and at some point there is no metal left under the brown. You recognise it because the rust has structure — pitted, raised, layered — and because a screwdriver, or even a thumb, pushes into it.

The practical line between them is simple: what can be brushed off and protected is cosmetic. What yields is structural. The difference is not the colour but the pressure of a finger.

Rust starts where water stands and where dirt holds moisture. On almost every car those are the same places, and one walk around the car covers them:

For the technical inspection and for safety, what counts is not how much rust there is but where. Load-bearing parts are a category of their own: sills, floor pan, longitudinal members, every mounting point for axles and subframes, the seat belt anchorages and the spring seats.

At those points, perforation is not a bargaining item but a reason to walk away. A proper repair there means cutting out, welding in and re-protecting — work that on an older car regularly costs more than the car is worth. Filled and painted over, it is not a repair but a postponement.

The distinction matters in the other direction too: a rusted-through wing is annoying and replaceable. A rusted-through spring seat is neither.

Two components deserve their own paragraph, because their failure is immediate. The first is the brake lines. They usually run along under the car, often in clips where dirt packs in. A line corroding there will keep holding working pressure — until the one stop when it does not. At a technical inspection, heavy corrosion on brake lines is grounds for a failure, not for an advisory.

The second is the spring seat. If a coil spring breaks at its lower end, because the seat gives way or the spring itself has corroded, the free end can reach the tyre. So on any car from a region that salts its roads, a look at the spring seats and at the lower coils belongs to the inspection — with a lamp, through the wheel arch.

Almost all serious body rust starts on the inside. Cavities are wax-protected at the factory, but the protection ages, and the drain holes meant to let water back out block with leaves, wax residue and dirt. After that, water stands in a cavity whose outside paint is perfect.

The second source is water paths that have nothing to do with rust until they block: sunroof drains, the drains under the windscreen, the door seals. The third is accident repair: wherever panels were welded or pulled, the factory coating is broken, and if the cavity was not re-protected afterwards, that is where it begins.

So the most useful question at a viewing is not “does it have rust” but “where has it been worked on”. Differing paint thicknesses, overspray on seals, a seam without the factory bead and bolts with twisted paint edges are the pointers.

On cars with aluminium panels — bonnets, doors, whole bodyshells — there is no brown bloom. Instead you see whitish-grey powdery patches and paint blisters lifting from below. It often starts where steel meets aluminium: bolts, rivets, hinges. Two dissimilar metals plus moisture make a cell, and the less noble metal gives way.

The repair is a different world from steel: aluminium is welded differently, needs different tools and, in many shops, a separate bay so that no steel dust gets in. If you buy a car with an aluminium body, find out beforehand which workshop within reach does that work at all.

You need little, but you genuinely need that little: a bright lamp, a small mirror or a phone with a front camera for the places behind panels, a cloth and clean hands. Everything else is comfort.

The order that works: first a pass at eye level over every seam and edge, then crouch down to the sills and arches, then empty the boot and the spare wheel well, then open the doors and look at the lower edges from inside, and finally — if there is any way — get it on a lift. Half an hour on a lift replaces every guess.

And use your fingers. A seam that feels rough where it should be smooth stands out more than any photograph. A magnet test says little on modern cars with plastic wings and aluminium panels; a thumb says more.

An evenly black-sprayed underbody on a ten-year-old car is not automatically an alarm — plenty of owners genuinely have preventive protection applied, and in regions that salt the roads that is sensible. But it always deserves a question, because the same layer hides what is beneath it.

What you do then: look at the edges where the coating runs out, and check there for blisters and flaking. Black compound sitting on a bulge often has a bulge of rust underneath. And ask whether there is an invoice — a shop that applies protection usually documents the condition beforehand.

Cars from regions that salt their roads in winter age faster underneath than cars from the dry south. Coastal air pushes in the same direction: salt-laden air reaches places grit never touches. That is one reason it can pay to look across borders when buying used — and one reason why doing so without your own inspection is worth nothing.

A car from the south may bring other issues instead: hardened rubber parts, cracked dashboards, an air conditioning system with many hours on it. Rust is one criterion among several — just the one with the most unpleasant consequences.

If you are searching across Europe, plan the inspection on site, either yourself or through a workshop near the car. The journey there is cheaper than a sill.