On 01/10/2017 19:06, Bill wrote:
I may be about to buy, from a scrapyard, a set of 5 replacement wheels
for the recently purchased car to get a spare wheel, proper sized tyres
and wheels that fit the wheel nuts.

In the past, I've often been told that the reason for a slow puncture
was that the alloy wheels had gone porous.
More recently, I've been told that the cylinder head on a certain
vehicle had gone porous and that was why it chuffed when cold.

I have always wondered whether this is a standard get-rid-of-him phrase
taught as part of the Car Mechanics PhD (Hons) course, or whether alloy
does actually start to leak.

Does anyone know? Is there any sort of standard test, like wheel
tapping, that can detect this sort of rot?

Paint them well before the tyres are fitted, they will probably be fine.
Testing would cost more than a new wheel.