Can you leave it, yes, should you leave it, no. It really should have a finish coat of paint.Regarding a new plastering job done to my garage converted into a studio, the plasterers did a swirl texture to the ceiling, does it need to be painted or can I leave the plaster as is?
If I need to paint it, can I use a roller or does it require a professional quality sprayer?
In regards to the walls, which I will paint, but have areas where there was compound used previously and also the new plaster, will basic water based primer and then paint be ok? I cannot use anything oil based due to issues to fumes.
Yes, a roller will work just fine, a 1/2 inch nap.
A GOOD latex primer will work, Zinsser's 123, or Gardz(if you can find it)
Be sure to wait the appropriate time for the plaster to cure( if it was actual plaster). Ask the people who did the job.