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I used solvent allcoat in white last year. Found it really horrid to work with - super smelly and really difficult to get a smooth finish. Couldn't apply and then lay off as would drag and be worse than the initial stroke. I tried warming it up too - but no improved result. So defo avoid if you can. I used it because it had a lower application / higher humidity rating for application than the water based and i was working through winter.
Gloss is harder wearing than satin. Not sure what to recommend for priming UPVC as I don't come across it very often in the properties I work on. I think Zinsser 123+ would do it but would need to check.
I would ignore the self priming claims of Allcoat and prime teh masonry with Zinsser 123+ anyway.