Rust on Classic Cars: Why It Starts and How to Fix It

Rust is hydrated iron(III) oxide, formed when the iron in steel reacts with oxygen and water. Take either away and the reaction stops, so keeping a classic dry matters most. Salt water speeds rusting up, and winter roads carry plenty: National Highways alone keeps around 280,000 tonnes of salt in its barns at the start of winter. Zinc changes the picture, because it seals steel and corrodes in preference to it. Porsche says the 911 became the first series production car with a body hot-dip galvanised on both sides in 1975. A car without that zinc layer relies on paint, sealer and wax.

The rust you can see is rarely the worst of it. Water and road dirt settle inside box sections, on ledges and under carpets, so the metal often fails from the inside out, and the inner sill where it joins the floor is a common example. On a monocoque such as the MGB, the inner sill, membrane and castle rail form a closed box that gives the body its longitudinal strength, so rotten sills are a structural fault rather than a cosmetic one. On a separate-chassis car such as the Triumph Herald, the outriggers that carry the body deserve the same scrutiny as the main rails.

View the car dry and in good natural light, because rain hides a great deal. Blistered paint on wheel arches, door bottoms and windscreen scuttles usually means corrosion working outwards from behind. Never go under a car held only on a jack; use axle stands or a hoist. MOT testers judge corrosion by finger and thumb pressure and by lightly tapping or scraping with a corrosion assessment tool, and they concentrate on load-bearing metal within 30 cm of steering, suspension, braking and seat belt mountings. An inexpensive illuminated endoscope, pushed through an access hole, shows the inside of a sill.

Structural rot has to be cut back to sound steel, then replaced with new metal of the right gauge and shape. The MOT inspection manual says a repair must appear as strong as the original structure. Spot welds are accepted only where the original panel was spot welded and has been removed; stitch or plug welds may stand in for them, and other patch repairs must be continuously seam welded. Gas brazing, soldering, adhesive bonding, fibre reinforcement and body filler are not acceptable on load-bearing members, and DVLA guidance also requires chassis and monocoque repairs to be continuously seam welded.

Once the metal is sound, the job is to stop water sitting on it. Wax-based cavity protection, sprayed through a long lance into sills, doors and chassis sections, is designed to creep into seams and displace moisture; reapply it wherever welding has burnt it away, and check it each year. Keep the drain holes in doors and sills clear, rinse salt from the underside after winter journeys, and touch in stone chips promptly, because any gap in the paint leaves steel open to air and water. Store the car somewhere dry and ventilated, and never put a cover over a wet car, which traps moisture against the paint.

Rust is cheapest to deal with early, so have the underside checked every year, ideally on a hoist, and after any winter the car has spent on salted roads. A good inspection looks inside the box sections as well as at the outer panels, and notes anything within 30 cm of steering, suspension, brake or seat belt mountings. Keep the report with the car's records. Classic And Retro Autos in Witton Gilbert, County Durham, offers a £65 plus VAT inspection of about an hour on the hoist, and welding and fabrication, including correcting poor previous repairs, from £80 an hour plus VAT, by appointment on 0191 3730086.

Surface rust on otherwise sound steel can be cleaned back and sealed, because rust needs both oxygen and water and an intact coating keeps them out. Once steel is holed or has lost thickness, no coating restores its strength. The weakened metal has to be cut out and new steel welded in, especially in load-bearing areas such as sills, floors and chassis rails.

Not if the rust makes it unroadworthy. A car over 40 years old that has not been substantially changed needs no MOT, but GOV.UK says it must still be kept roadworthy, and using a vehicle in a dangerous condition can mean a fine of up to £2,500 and three penalty points. The MOT manual's corrosion standard is a sensible benchmark.

Not on structural metal. The MOT inspection manual rejects gas brazing, soldering, adhesive bonding, fibre reinforcement and body filler on load-bearing members. Plating or welding must reach sound metal, and patch repairs generally have to be continuously seam welded. Filler belongs only as a thin skim over sound, properly welded panels.

Start with the sills, particularly the inner sill where it meets the floor, then the floorpans, wheel arches, door bottoms and windscreen scuttle. On a separate-chassis car, check the chassis rails and outriggers. Rust often begins inside box sections and on ledges where water and mud collect, so bubbling paint outside usually means more damage within.