In my experience white primer from Halfords never used to be "soft" -until the last few cans I've had anyway. In any case I can tell you most of Halfords rattle cans have been acrylic for a very long time. proper paint (cely) was banned in Scotland a very long time before England -it's been a nightmare to get proper paint up here for a good while unless you were commercial.
FerryNostalgic
...Did you get any scotchbrite? It's brilliant at cutting back round detail. It may save you allot of hassle -particularly on leaving paint as-is. It's as rough or delicate as you buy it and use-it. It may save you allot of hassle!
Worth noting also humidity is a spray painting killer, possibly more with airbrush than rattle can. You need a reasonably dry (and warm) environment) to spray. I have a relatively cheap dehumidifier I use for drying clothes, my last one lasted 10 years constantly drying clothes and setting up rooms for spraying. Can't complain and no complaints of orange peel -varnish clouding.
Hope this helps,
Rich