I do plenty of skimming. I'd Stanley lines/crosses all over it . Then scrape the flakey bits off , if you don't then theirs a good chance the weight of the new plaster will pull the flakey stuff off along with the flakey plaster . Then paint blue grit all over the ceiling before skimming . We used to PVA but moved to blue grit and haven't looked back , it's only 1 coat and the aggregate is a amazing key , plus the blue colour shows you where you've painted so theirs no missed bits