What is the best way to go about smoothing a painted popcorn ceiling? Scraping it is much too hard, so I guess a skim coat is the way to go, but what is the best material to use for the skim coat?
Is it also possible to do a skim coat over unpainted popcorn, without scraping it?
Jeff, taking a ceiling from popcorn to a smooth surface is no easy task, no matter how you look at it. I laid out all the option in this guide to remove popcorn ceilings, but suffice to say even in the best circumstances, the drywall surface left once that popcorn is removed will still be quite rough.
If you truly want a new, smooth ceiling, the best approach is to add a 2nd layer of drywall to the old ceiling. This may sound like a lot of work, but I think it's less work than removing the ceiling, or trying to smooth over what's left with more plaster. You can use thinner, 3/8th inch drywall too and to avoid having to spackle the seam between the walls and the ceilings, consider crown moldings.