Good stuff Geo.
Judging by the approximate time you obtained the Humbrol paint, I think it will be a fault of the paint, not your use of it. At that time, there were many problems, including paint which would not dry at all, and could exhibit a 'rubbery' finish.
These problems have persisted, along with others, to the point that Hornby products, the owners of the Humbrol brand name, have now moved production back to the UK.
Since the demise of Humbrol as a company, and the closure of the Humberside factory, the paint was made outside the UK, with, as we've witnessed, many, many quality problems. Hopefully, this will now cease, and we can expect to see the Humbrol brand back where it used to be, at the top.