Aquapanel skimming?

[drawis]

Morning all,

So I have a small (c.1m2) area of aquapanel which has been fashioned into a drop down ceiling box for an extractor fan.
(I used aquapanel becasue it is moisture and heat resistant and anyway it's too late to change it!)

I'm intending to skim it (and ultimately paint it) with some Touprec Joint Skim & Fill that I have. My query is whether the boards will need some priming first- everything I see online says they are very thirsty and can benefit from doing so. I've also read that pva is a no go (a youtube video shows somebody's plaster needing scraping off after he used pva- he neded up using Mapei G but then he was using proper plaster).

So- do I prime. What with?

Thanks all