Humbrol has been banished from my household for a good number of years now so can't comment but:
A few years ago I could not service the 4-pot brake calipers on my Celica GT4. After all sorts of attempts including roasting hot with a flame, boiling in hot water, dunking in diesel to release the cylinders inside, pretty much the only thing that survived was the smoothrite paint on the calliper body. And I do mean I roasted the caliper to oblivion with a direct hot rosebud for many minutes as much I dared before melting the aluminium and the smoothrite just shrugged it off!
I used to paint various car parts with expensive and specific VHT but they tarnished. Many a time I stripped them off, re-painted in smooth-rite and never had a reason to look at them again as they took acid cleaners also when the VHT didn't like them. I don't know what is in hammerite's formula but their smootrite is great stuff. No doubt it changed since, but I've not used any VHT branded paints for a while now as smoothrite seemed to last better. It might be worth a try for you?