Ceiling in foyer had been painted or sprayed during a reno. Not sure which and also not sure with what surface paint. So to smooth out “mud” was applied over to remedy previous job. When mud dried there were bubbles or blisters throughout. What caused this and what should we do?
Cause- The surface wasn't primed with a good primer before applying the drywall mud. Zinsser Cover Stain or Zinsser 123 work well. This is a problem that has occurred for decades with a lot of discussion. I have found using a primer first usually prevents the bubbles.
Fixing- First scrape to open up the bubbles. Prime the area, use an oil based primer as mentioned. Now skim a coat of drywall mud over the craters, sand smooth after dry, texture if needed and repaint. It is a good idea to prime again before painting, but if a good acrylic paint is used you can skip priming and apply 2 coats.