A tunnel brush is a cloth or foam finger spinning at speed. On the first car of the day it is clean. By the tenth it is carrying grit from nine other cars — brake dust, road salt, sand from a coastal run — and it is pressing all of it into your clear coat at somewhere near a thousand revolutions a minute.

On silver you will never notice. On black, dark blue or dark grey you will notice within three washes. What people call fading is thousands of shallow scratches catching light at different angles, so the panel scatters light instead of reflecting it. The paint has not changed colour. It has stopped behaving like a mirror.

What actually removes dirt

Chemistry and water, in that order. A pre-soak breaks the bond between the film of dirt and the paint. A second, thicker foam lifts it clear of the surface. Only then does anything touch the panel, and when it does it should be a fresh microfibre cloth that has never met another car.

If a cloth has already been used on one panel, it is now a tool for adding scratches, not removing dirt.

How we do it on a driveway

• Two-stage foam, with a dwell time long enough to matter

• Deionised water, so there is nothing left behind to dry into spots

• A fresh microfibre per panel, laundered at the end of every day

• Wheels, arches and sills last, with separate cloths that never go near paint

It takes twelve minutes instead of four. That is the whole trade. You are not paying for speed, you are paying for the paint to still look like paint in three years.

If the damage is already there

Swirls can be removed. A clay bar pulls out what is bonded to the surface, a light machine polish levels the clear coat, and a coat of wax or ceramic gives you a few months or a few years of protection depending on which you pick. On most cars that is a single ninety-five minute Full Detail — and then a fortnightly wash keeps it there.