This week I am indebted to an Autocar reader who asked for my opinion of paint depth gauges.

If you have never come across one, it is a magical little device that the pros use to tell them in an instant how many microns of paint are on each panel. The paint is dead even when the car first comes from the factory, but if there has been a respray, then you can tell in a few seconds and investigate further.

After several bad experiences of buying cars privately and from dealers, the reader wondered whether or not he would be better armed with one of these devices. At the cheaper end of the scale, they are still more than £200. I don’t think that it’s possible to justify one and I buy, on average, two or three cars a year.

If I had used a paint depth gauge on my BMW Shed 7, for example, I probably wouldn’t have bought it. Sometimes you can know a bit too much about the used car you’re about to buy.

Here’s the thing: buying a used car is 90% absolute luck. The remaining 10% is you taking the time to take a few precautions. The simple fact is that a car can go wrong the day after you’ve bought it. That’s just the nature of the used car gods, who can be fickle at the best of times.