I removed painted popcorn, scrap the painted popcorn first with a scraper. The perlite comes off in balls. Then spray with solution of fabric softener and water. Let it soak for a while. Mine came off (the brown thick paint came off in sheets), 15 x 11 room took me 2 days. Tape off room with painters plastic. Scrape ceiling with trowel. spray ceiling with water solution, wait 10-15 minutes to soak through. Watch out for overworking an area, you'll gouge it. Then use Magic Trowel to skim coat the ceiling. Here is a link that might be useful: Great tool for DIY skim coat

The big question is - does it contain asbestos or not? You can look in the yellow pages and find asbestos testing places. Or you can look the other way, which I do not recommend, based on experience. I had an estimate of $600 for a 15 x 25 foot living room/dining room - decided to do it myself. I got a lung infection from the dust, developed pneumonia, etc. Doctor bills plus lost time from work was way over the $600. (Antibiotic pills were $400 for 10 days.) Also, the hugest mess you ever saw, anywhere. But, if you are really a die hard DIYer, please invest in a respirator, not a little painting mask. Good luck!

Nice job of removing the acoustic! Trying to get a smooth finish on drywall is tough because the paper is more porous than the mud joints and will show no matter how finely feathered the edges. One way is to size the entire surface lightly by applying your mud, but wiping off as much as possible. This is similar to skim coating with the exception that you are not leaving a thin layer of mud to try and get an even surface. This is best accomplished with skim coating plaster over blue board.The advantage that you have ,however, is that your ceiling is already somewhat sized because of the acoustic being sprayed on. Even though you have removed the texture, the mud base of the acoustic is still on and adhered to the dry wall. You might give it a go yourself by filling any imperfections, sanding smooth the entire surface.Any bare drywall spots that might present their self you can size as explained. Start with a small area of the cieling and see how you feel about it.If you have the confidence, move ahead. If not, get a pro.

joe mn : Likely mould in the walls. Especially if it was a vinyl wallpaper, which prohibits inward drying of moisture. OP: Something wrong with the product applied. Or else it's a real world example of thepost hoc ergo propter hoc fallacy.