Water damaged new plaster

[Keo223]

I wonder if anyone can give me some advice on this. Recently had an extension built by a joke of a company. National, well-known, won’t name names. It’s suffered six leaks through glass roof due to panels being fitted incorrectly and no sealant applied, but is now thankfully water tight and I’m wanting to paint.

I’ve 90-10 coated the walls which were not damaged but left the damaged part of ceiling to dry out. It now seems dry but has a water stain on the skim. Do I just go same as the walls or do I need to seal with a bought product first?

This four week job has now been going seven months so am keen to get it finished and get it right first time.

Thanks