Mechanical parts can be rebuilt. A rotten structure is a different matter, because repairing it means cutting and welding, often by a specialist, at a cost that can exceed the value of the car. That is why rust decides the price of a classic car more than anything else.
Where rust starts
Rust starts where water and dirt collect and where air cannot dry the metal.
- Sills, door bottoms, wing bottoms and the lower rear quarters.
- Floor pans, especially under the carpet and around the seat mountings.
- Wheel arches, inner wings and strut tops.
- Boot floor, spare wheel well and the area below the rear window.
- Around the windscreen and the rear screen, where seals fail.
- Seams and flanges where two panels meet.
- Chassis rails, sub-frames and suspension mounting points.
How to find it
- A torch. Look into every gap, behind trim and under the car. Bubbling paint is the sign that rust is coming from behind.
- A magnet. A weak magnet or a fridge magnet in a cloth sticks to steel and not to filler. If it drops off at a lower panel edge, there may be body filler there.
- Tapping. A screwdriver handle tapped on a sill sounds solid on steel and dull on filler.
- Pressing. Press gently on suspect places. If the metal flexes or crackles, it is rotten.
- A lift. Many problems only show from below.
- Photographs. Take pictures of every suspect spot so that you can compare notes with an expert.
Surface rust and structural rust
- Surface rust is on the paint or bare metal surface and has not weakened the metal. It can be removed and treated.
- Penetrating rust has eaten holes in the panel. It needs cutting out and replacing with new metal.
- Structural rust affects the parts that carry loads: sills, chassis rails, suspension mounts and the floor edges. This is the serious kind, and in many countries it affects whether the car passes inspection.
A proper repair versus a cover-up
A proper repair cuts out the rotten metal and welds in new metal, then treats the inside and the outside. A cover-up fills the hole with filler or glass fibre mesh, paints it, and hides the problem for a year or two. Signs of a cover-up: thick filler, a panel that is too smooth, new paint over old rust bubbles, fresh underseal over everything, and a magnet that slides off.
Ask what was done, by whom, and whether there are photographs of the work in progress. A history file with pictures of a restoration is worth a lot. See papers and proof of ownership.
How to think about the cost
- Get a quote from a body shop that knows classics, before you buy a car that needs welding.
- Add the costs of paint and preparation. Welding is only the first step.
- Compare the total with the value of a good example. If the repair costs more than the car is worth, you need another reason to do it. See planning a restoration.
Protecting the car afterwards
- Keep it dry and ventilated. See winter storage for the principles, which apply to cars too.
- Clean the drain holes and gutters so that water can leave.
- Wash off road salt and dry the car.
- Consider cavity wax inside sills and doors and underbody protection that is applied to clean metal.
