Direct answer: Defenders corrode because their aluminium alloy skins are fastened to a steel skeleton — bulkhead, door frames, pillars, seat box, crossmembers — and wherever the two metals touch with moisture present, galvanic reaction eats the joint from the inside. The alloy panel bubbles and the hidden steel dissolves, which is why a Defender can look sound and be structurally rotten.
The chemistry in one paragraph
Aluminium and steel sit far apart on the galvanic series. Join them, add electrolyte — road salt spray is ideal — and you've built a battery: the aluminium sacrifices itself at the joint while trapped moisture rusts the steel side simultaneously. Factory insulation between the metals degraded decades ago on every Defender now on the road.
Where to inspect
Bulkhead: footwells, door-pillar faces and the top corners under the vent flaps — the single most expensive structure to ignore; repair sections are in firewalls & cowls.
Door frames and bottoms: steel frames under alloy skins — check the bottom hem and hinge mounts; replacements in doors.
B and C pillars: bubbling paint lines at pillar bases; sections in pillars.
Sills and floor supports: probe from below; assemblies in floors & sills.
Fender-to-bulkhead flanges: white oxide powder at the joint line; fender assemblies replace the lot.
Rear crossmember and boot floor: the classic MOT failure.
Repairing it properly
Replace the affected structure — patching galvanically-eaten steel invites the same failure at the patch edge — and rebuild the joint the way the factory should have: isolate alloy from steel with barrier tape or sealant, use coated fasteners, and seal joints so electrolyte can't enter. Our panels follow factory material choices (alloy where original, steel where structural) so repaired joints behave like the originals, isolated properly.
Era matters
Tdi (1990–1998), Td5 (1998–2007) and Td4/Puma (2007–2016) trucks differ in bulkhead, dash, seat box and door construction — match the era stated on every listing, or send us a VIN to confirm.
Prevention after repair
Cavity wax every enclosed steel section annually, keep drain holes clear, wash salt out of the chassis and joints in winter climates, and re-seal any joint you disturb. A Defender maintained this way outlasts its owner.
Browse: all Defender panels · 90 · 110