Car Rust Explained — What's Fixable and What Isn't?
Rust is the slow-motion killer of used cars. Some rust is cosmetic; other rust is terminal. Here is how to tell the difference and when to walk away.
Types of Rust — Fixable vs Terminal
Surface Rust (Fixable)
Surface rust appears as orange or brown discoloration on paint or metal. It has not penetrated through the metal. Appearance: rough, powdery, flaking — but the metal underneath is intact.
Test: Tap it with a hammer. It should sound solid. Scrape it with a screwdriver — if it easily flakes away, it is surface rust.
Cost to fix: £200–£600. Lightly sand, apply rust converter, prime, and paint. This is a minor cosmetic fix.
Perforation Rust (Terminal)
Perforation rust has eaten completely through the metal, creating holes. It typically occurs in the floor pan, sills, or wheel arches where water collects.
Test: Press the area with a screwdriver. If the metal collapses or you push through to daylight, it is perforated.
Cost to fix: £1,000–£5,000+. Requires welding new metal panels. This is expensive and requires specialist skills.
Structural/Chassis Rust (Don't Buy)
Rust affecting the chassis, sills, or load-bearing panels is structural damage. The car's strength and safety are compromised.
Cost to fix: £3,000–£10,000+. Often not economical. Many insurance companies and MOT testers will fail the car.
Where Rust Occurs — Priority Check Points
Floor Pan (High Risk)
The floor pan is where water collects and rust starts. Get under the car and inspect it with a torch. Look for holes or thin spots. If the floor has rust, expect structural work.
Sills (High Risk)
The sills run along both sides under the doors. They are structural and support the car's weight. Rust here is serious. Push on them with your hand — they should be solid, not flexible.
Wheel Arches (Medium Risk)
Water and road salt collect in wheel arches. Surface rust is common and fixable. Perforations require panel work (£500–£1,500 per arch).
Door Panels and Boot Lid (Low Risk)
Rust on doors and boot lids is typically cosmetic. If perforation, panels are easily replaced (£300–£800 per panel).
How to Inspect for Rust
1. Visual Inspection
Look at all surfaces: paint, gaps between panels, under the trim. Orange/brown discoloration means rust is present.
2. The Tap Test
Use a small hammer and tap suspect areas. Solid metal sounds like a "ding." Rusted/thin metal sounds hollow or dull. Heavily rusted metal may sound like a thud or crackle.
3. The Screwdriver Test
Press a flathead screwdriver into the rust. If it sinks in easily or breaks through, the metal is compromised.
4. Get Under the Car
This is essential. Look at the floor pan, sills, and chassis with a torch. Bring the car to a garage and ask if they can do an underside inspection (£50–£100).
Red Flags That Mean Walk Away
- Perforations or holes in the floor pan
- Soft or rusty sills
- Fresh underseal applied to hide rust (shiny coating over aged panels)
- Multiple rusted areas across different parts (systemic problem)
- Sellers trying to hide rust with underseal or paint (dishonest)
Preventing Rust on Your Used Car
- Wash regularly: Salt and dirt accelerate rust. Weekly washes in winter are essential
- Check drains: Blocked door/bonnet drains trap water. Keep them clear
- Underseal: A good underseal coat (£300–£600) protects the chassis for 3–4 years
- Fix damage quickly: Scratches and chips expose bare metal. Touch up with paint immediately
Check the free MOT history before viewing any used car. MOT records show whether the car has had advisories or failures related to corrosion — giving you an idea of its rust history before you travel to see it.