Gritters go out earlier than most people expect, and on the higher routes they are working well before anyone has thought about winter. A hard frost on the tops does not need snow to bring the lorries out, and once the salt is down it stays on the road surface, gets kicked up as brine by every vehicle, and coats the underside of anything that drives through it.

The chemistry is simple enough. Salt dissolved in water makes the water conductive, which is exactly what steel needs to rust quickly — corrosion is an electrochemical process and salt water accelerates it dramatically. Worse, salt is hygroscopic: it holds onto moisture. So a car parked up after a salted drive is not drying out. It has a damp, salty film underneath keeping the reaction going for days, whether it moves or not.

What Corrodes First

Not the panels. The underside, in a fairly predictable order.

Brake pipes. The most serious item on the list, covered properly below. Steel lines running the length of the car, frequently in the path of spray thrown off the front wheels.

Fuel lines and fuel tank straps. Same exposure as brake pipes, less immediately dangerous but no fun to deal with, and a corroded tank strap is an MOT matter as well as an obvious problem.

Subframes, sills and chassis members. These are usually boxed sections, which is the problem. Salty water finds its way inside through drain holes and seams and cannot get out, so the corrosion happens on the inside where nobody sees it. A sill can look sound and be paper thin, and corrosion near a suspension or seatbelt mounting is a straight MOT failure.

Suspension mounts, spring pans and rear beams. Spring pans on the rear of many cars collect road debris and hold it damp against the metal. They rot from the inside of the dish outwards, which is why they are a frequent find.

Exhaust systems and their hangers. The system itself often outlasts the brackets and rubber mounts holding it up. A snapped hanger lets the system hang and rattle and eventually break at a joint.

Brake calipers, slide pins and handbrake mechanisms. Salt gets into slides and cables and seizes them. That is why sticking calipers and binding handbrakes are far more common after a winter than before one.

Electrical connectors and earth points. Nothing dramatic, just a long, slow source of intermittent faults. A corroded earth causes the sort of odd electrical behaviour that is maddening to chase and easy to prevent.

Why Brake Pipes Are The Serious One

Everything else on that list is an inconvenience or a repair bill. Brake pipes are a safety issue, and they deserve treating differently.

The pipes carry hydraulic pressure from the master cylinder to each corner. They are steel, coated when new, and they run underneath the car where they take the full force of wheel spray, grit and salt for the whole of their life. When the coating is breached, they corrode from the outside inwards, gradually losing wall thickness.

The problem is that this happens without any symptom whatsoever. The brakes feel completely normal right up to the moment a pipe gives way, and when it goes it does so under pressure — typically at the moment you brake hard, because that is when the pressure is highest. You lose part of the braking system instantly, with no warning at all.

They are also difficult to assess casually. Surface rust on a pipe is not necessarily a problem; heavy flaking corrosion or pitting is. Judging that means getting underneath with a light and looking properly, including along the runs that pass behind shields and above the fuel tank where they are awkward to see.

Washing The Underside Matters More Than The Paint

Most people wash a car because of how it looks, so they wash the bit they can see. For longevity, that is the wrong way round. The paint on modern cars is durable and galvanised panels resist rust well. The underside has far less protection and takes far more punishment.

What actually helps:

• Rinse the underside regularly through the salted months. Plain cold water is enough — the aim is dilution, getting the salt off before it sits. Once a fortnight in a hard winter is worth far more than a detailed wash in April.
• Get water into the arches and up behind the wheels, where spray is thrown hardest and mud packs in. A hose with a decent nozzle does the job; a jet wash lance angled up into the arches is better.
• Clear out the drain holes. Sills, doors and boxed sections have drains that block with leaves and road grit. Blocked drains mean water sits inside the section — that is how cars rot from the inside out.
• Do not park immediately after a salted drive without rinsing, if you can avoid it. A car standing with a salt film underneath is corroding the whole time it sits.
• A garage is not automatically better. A warm, unventilated garage with a wet salty car in it can be worse than a cold, breezy driveway, because it keeps everything damp for longer. Ventilation beats warmth.

Underbody protection treatments and cavity waxes are worth considering on a car you intend to keep. They work best applied to metal that is clean and dry and already sound — spraying wax over active flaking rust just hides it. If the car is already corroding, deal with that first.

What An Autumn Underbody Inspection Finds

Done before the salt goes down rather than after, a proper look underneath is one of the more useful things you can have done to a car in this part of the country. What it turns up:

• Brake pipes and flexible hoses assessed properly, along their whole run
• Fuel lines, tank straps and shields checked for corrosion and security
• Sills, subframes, spring pans and chassis members inspected, including the MOT prescribed areas around mountings
• Suspension bushes, ball joints, springs and shock absorbers checked for wear, leaks and cracks
• Exhaust system and all its hangers and mounts checked
• Calipers and slide pins checked for free movement, and the handbrake mechanism checked and lubricated
• Blocked drain holes cleared while the car is up in the air
• Leaks identified — anything dripping oil onto a component is also stripping its protection

The value in doing it now is options. Something found in October can be fixed when it suits you, at a sensible price, on a car that is still on the road. The same fault found in February is found because it failed an MOT or because something broke, and neither of those is a good time to be making decisions.

If the car has done a few winters on gritted roads and nobody has looked underneath it, it is worth a look. We do underbody inspections as part of our servicing and repair work, wherever the vehicle is parked, and anything needing more than a mobile setup gets booked in with us.