Spotting rust on a car: which areas are critical and what they mean

Why rust matters so much when buying a used car

Rust is one of the most common — and at the same time most frequently underestimated — defects on used cars. Many buyers look at the engine bay, the interior and the mileage — and overlook the areas where rust can do the most damage over the years.

Yet not all rust is the same. There's rust that forms within a few hours on a brake disc that hasn't been used for a week — and rust that spreads through cavities over years until it has destroyed load-bearing structures. Anyone who treats both the same will assess a vehicle incorrectly. Anyone who recognises neither may end up buying a pig in a poke.

Surface rust: harmless, but not a free pass

Surface rust forms anywhere bare metal is briefly exposed to air and moisture. Typical spots:

• Brake discs: An orange-red coating after a few days of standing is completely normal. It is worn off the first time you brake.
• Exhaust system: Light surface rust on the outer shell is normal on vehicles that do little short-distance driving. More relevant is rust at the transitions between the manifolds, centre silencer and tailpipe — there it can lead to rust-through, forcing a replacement.
• Struts and shock absorbers (exterior): Light rust on the outer shell is harmless. Rust on the piston rod (the shiny part that slides into the damper), on the other hand, is a defect.

Surface rust is no reason to reject a vehicle — but it is a reason to look more closely. Where there is visible rust, there can be hidden rust behind it.

The critical areas: where rust becomes dangerous

Sills

The sills run as hollow profiles lengthwise beneath the doors. They are part of the body's rigidity and the side-impact structure. Because they sit close to the ground, they are permanently exposed to spray water, road salt and dirt.

What you can see: dents, bulged areas, cracks in the paint layer or already visible pitting on the lower edge. Pressing a finger on a suspicious spot (no force, just light pressure) sometimes reveals a hollow sound — a sign of advanced rust inside.

What you often don't see: the rust inside the hollow profile. There it eats its way from the inside out — by the time it becomes visible on the outside, it is often already well advanced inside.

Wheel arches

The wheel arches — both the metal behind the wheels and the transition area to the body — are among the most vulnerable areas of all. Dirt and water build up behind plastic trim and fender flares, and road salt accelerates the process.

What you see: blisters under the paint, exposed rusted metal, brown discolouration in the transition area. Some sellers spray over rust blisters with underbody coating — watch out for freshly applied, uneven layers in unusual spots.

Underbody

The underbody is the largest surface exposed to rust and is only partly accessible for a layperson's visual check. Without a lift or pit, you'll see at most the front and rear areas — and only with sufficient ground clearance.

Critical areas on the underbody:

• Longitudinal members: Run parallel to the direction of travel and form the backbone of the floor assembly. Rust here is a serious safety problem.
• Cross members: Connect the longitudinal members and absorb forces from the axles and suspension.
• Axle mounts and spring seats: Rusty mounts can loosen bolts and brackets.
• Fuel lines and brake lines: Corroded lines can start to leak and pose an immediate safety hazard.

What you can do yourself within limits: with low sun and a bright surface, crouch down and look flat behind the vehicle. A torch or phone flashlight helps to illuminate the directly visible areas. You won't get more than a rough impression this way.

Door edges and lower door edges

The lower edges of the doors are vulnerable because water can penetrate and stand in the cavity. Blocked drainage openings at the bottom of the door are a common cause: dirt clogs the small holes, water builds up — and the metal rusts from the inside.

Visible from the outside: blisters in the paint layer, lifted paint on the lower edge, brownish discolouration. Check all four lower door edges systematically.

Strut towers

The strut towers are the mounting points for the front strut struts in the wheel housings — usually clearly visible when you open the bonnet. They carry forces from the suspension and are part of the crash structure at the front.

Rust on the strut towers is rare but serious. It shows up as blisters in the paint, exposed rusting metal or — at an advanced stage — already visible corrosion scars in the metal. Such findings are a serious defect that directly affects roadworthiness.

Exhaust and exhaust system

The exhaust lives in a harsh environment: heat, condensation from the inside, spray water and salt from the outside. Light external rust is normal; more relevant is rust on welds, flanges and transitions. Rusted-through spots in the exhaust system are a defect relevant to the TÜV and can also affect the emissions measurement.

How to proceed as a layperson

A structured visual check for rust takes about 15 to 20 minutes in daylight. You can check the following without any tools:

1. Walk around the exterior in good daylight: scan all four wheel arches, lower sill edges, lower door edges, tailgate and boot opening for blisters, cracks or exposed metal.
2. Open the engine bay: check the strut towers and the transition area between the engine bay and the inner fender.
3. Underbody view: crouch down behind or in front of the vehicle and use a torch to illuminate the front and rear underbody areas.
4. Watch for fresh spray: an uneven, thick application of underbody coating in individual spots can indicate fresh reworking.

What you can't reliably assess as a layperson: the condition of the inside of hollow profiles, the exact extent of rust under sealing layers, and whether load-bearing structures are permanently affected. These questions call for an appraiser.

When a professional is essential

An independent vehicle appraiser sees the vehicle with different tools and a different depth of experience. checkdenwagen inspects the vehicle right at the seller's location — including a systematic visual check of the bodywork and underbody, an assessment of rust findings according to their structural relevance, and documentation in a photo report.

The result is not a guarantee — whether rust is already active inside hollow profiles is something even an expert can't determine with absolute certainty without disassembly. But the assessment is far better founded than a layperson's visual check. And the report provides a factual basis for the purchase decision or the price negotiation.

Used-car inspection from €289 incl. VAT and travel — right at the seller's location, throughout Germany.