We have removed the wood chip from our hallway ceiling - hooray! However there is an old layer of grey (paint?!) left all over the ceiling. There are a couple of patches where the grey has chipped off and they Will be obvious when painted over . I just want to Be able to paint the ceiling! Please tell me I don’t have to scrape off all the grey?! Would a thick undercoat (the kind that covers hairline cracks) work?