Where does the MGB rust?
The MGB's strength lives in its sills. Cosmetic rust on wings and doors is common and fixable, but rotten sills, castle rails or spring hangers turn a project into a full restoration.
Never jack or cut out sills on an MGB without bracing the door apertures first: the shell can sag and the doors will never fit again.
The 3D model could not be loaded. The notes below still apply.
Tap a pin or an area below to see where it rusts. Drag to turn the car.
- Structural: affects strength
- MOT failure risk
- Cosmetic, but it spreads
The model is an MGB GT; the roadster rusts in the same places.
Where it rusts, worst first
Sills (outer, inner and castle rail)
- What to look for
- Bubbling along the outer sill, a soft or crunchy castle rail underneath, door gaps that narrow at the top when the car is jacked.
- Why it rusts here
- Three-piece box section with poor drainage that fills with mud and water from the front wheels.
- Repair
- Full sill replacement with the shell braced. Specialist job on most cars.
Kit for this job
- Dinitrol ML Penetrating Cavity Wax, 2 x 500 mlBest overall in our cavity wax test; thin enough to creep into folded seams and it stays activeBuy on Amazonour test
- U-POL Weld#2 Zinc Weld-Through Primer, 450 mlOn every joint face before a panel goes in; it conducts, so you weld straight through itBuy on Amazonour test
- VELPAX 140A MIG WelderBest value all-rounder in our welder test, if you are doing the panel yourselfBuy on Amazonour test
- U-POL Tiger Seal, grey, 310 mlOver every new seam before paint; stays flexible and takes paintBuy on Amazonour test
Rear spring hangers
- What to look for
- Flaking or holed metal around the front leaf-spring mounting points ahead of the rear wheels.
- Why it rusts here
- Road spray hits them directly and they sit in a box section that rarely gets treated.
- Repair
- Weld-in repair sections available; check surrounding floor while there.
Kit for this job
- Bilt Hamber Deox-CFor mounts and brackets you can take off and soakBuy on Amazonour test
- Hycote High Zinc Primer, 400 ml (pack of 2)Over 60% zinc in the dry film; on bare, clean metal onlyBuy on Amazonour test
- Dinitrol 4941 Black Underbody Wax, 3 x 500 mlBest overall in our underbody test; a wax film that stays flexible, where bitumen cracks and traps waterBuy on Amazonour test
Battery boxes
- What to look for
- Holes or heavy scale under the rear seat area where the twin 6V batteries sit.
- Why it rusts here
- Battery acid plus water ingress from under the car.
- Repair
- Replacement boxes available; often found alongside rear floor rot.
Kit for this job
- Evapo-Rust C700Best overall in our remover test; strips rust to bare metal without harming sound steelBuy on Amazonour test
- Jenolite Rust Converter SprayBest overall in our converter test; the spray covers wings, arches and floors fastBuy on Amazonour test
- Hycote High Zinc Primer, 400 ml (pack of 2)Over 60% zinc in the dry film; on bare, clean metal onlyBuy on Amazonour test
Floor pans and boot floor
- What to look for
- Damp carpets, soft spots underfoot, holes in the boot floor around the spare wheel well.
- Why it rusts here
- Leaking door and screen seals keep carpets wet against the metal.
- Repair
- Patch or full panel. Cheap panels, labour-heavy fit.
Kit for this job
- Bilt Hamber Hydrate 80Best value in our converter test; for floors you cannot strip bareBuy on Amazonour test
- Dinitrol 4941 Black Underbody Wax, 3 x 500 mlBest overall in our underbody test; a wax film that stays flexible, where bitumen cracks and traps waterBuy on Amazonour test
- U-POL Tiger Seal, grey, 310 mlOver every new seam before paint; stays flexible and takes paintBuy on Amazonour test
Front wings
- What to look for
- Rust around the headlamp bowl, along the top seam and at the bottom rear corner behind the wheel.
- Why it rusts here
- Mud traps behind the headlamp and at the rear wing-to-sill joint.
- Repair
- Repair panels available; welded wings mean cutting at the seam.
Kit for this job
- Jenolite Rust Converter SprayBest overall in our converter test; the spray covers wings, arches and floors fastBuy on Amazonour test
- Hycote High Zinc Primer, 400 ml (pack of 2)Over 60% zinc in the dry film; on bare, clean metal onlyBuy on Amazonour test
- Dinitrol ML Penetrating Cavity Wax, 2 x 500 mlBest overall in our cavity wax test; thin enough to creep into folded seams and it stays activeBuy on Amazonour test
Door bottoms
- What to look for
- Bubbling along the lower edge and blocked drain holes.
- Why it rusts here
- Water runs down inside the door past the window seal.
- Repair
- Door skins or lower repair sections; clear drains and wax the cavity.
Kit for this job
- Dinitrol ML Penetrating Cavity Wax, 2 x 500 mlBest overall in our cavity wax test; thin enough to creep into folded seams and it stays activeBuy on Amazonour test
- Nitromors Rust ConverterFor seams and box sections you cannot reach with a brushBuy on Amazonour test
Inspection tips
- Check door gaps with the car on its wheels, then again with a jack under the sill. Movement means weak sills.
- Lift the carpets. A dry interior on a wet day is a good sign.
- Look for fresh underseal over the sills: it often hides repairs or rot.
Inspection kit
- OSRAM LEDinspect Pocket 200Slim, bright and rechargeable; it reaches behind trim and up into archesBuy on Amazonour test
- Blue Spot Tools 07327 telescopic magnetWeak enough to drop off over filler, which is the point; the telescopic stem reaches under sillsBuy on Amazonour test
- Rolson telescopic inspection mirror with LEDsFor the back of sills and inside archesBuy on Amazonour test
- Draper soft-grip awl, 75 mmPressed, never stabbed, along sills and floors; a thin screwdriver you already own does the same jobBuy on Amazonour test
Related guides
Frequently asked questions
Where does an MGB rust first?
Usually the sills and the bottom rear corners of the front wings, followed by door bottoms and the rear spring hangers.
Is a rusty MGB worth restoring?
Usually yes. Panels and Heritage shells are widely available, but structural sill work is the main cost driver, so price that in before buying.
3D model: MGB GT by Configcars / maxipub, CC BY 4.0; simplified for the web, markings removed.