Repost from my now defunct home blog. Original post date May 2007:
Oh, the popcorn ceiling. Popular treatment in the 1950s-70s. Builders liked it because they could spray it on and not worry about finishing the ceilings, homeowners… well, I sort of wonder if anyone ever really liked popcorn ceilings. After a while they become dirty and dingy since you can’t wash them. Some people try to solve the problem by painting over the popcorn, in turn creating another interesting mess since wet popcorn tends to fall off. We decided enough was enough and embarked on a journey to de-popcorn our basement ceilings.
Step one: Buy a pressurised sprayer at Home Depot/Walmart/any gardening center. Usually they are used to spray fertilizer and/or weed killer, but we used it to spray plain, old water. Fill sprayer with warm water and spray onto the ceiling. Wait five minutes. DO NOT OVERSPRAY! You can damage the drywall behind the popcorn, and peel off the backing paper.
Step two: Take a scraper, anything flat surfaced that you can scrape smoothly across the ceiling (we used 6″ wide spackle knives [at least I think that's what they are called]), and run it along the ceiling. The popcorn, at this point the consistency of spit balls, will fall right off. I should say at this point that it is a good idea to lay down plastic all over your floor. This is so messy that the spit balls fly everywhere when you scrape.
Step 3: Take ridiculous pictures of your H wielding the sprayer and the impressive popcorn stalactite hanging from the ceiling. Please people, do not paint over your popcorn. We lucked out. I’ve heard horror stories of painted popcorn being hellish to remove, but we didn’t have any problems other than it wanted to peel off in sheets.
Step four: (At this point I should mention that it would probably be wise to texture your ceilings after the popcorn has been removed. We chose not to do this step since we didn’t have the time/money/inclination to do so.) What you are left with is unfinished dry wall. There is a reason the builders sprayed the popcorn, they didn’t have to finish/fix dents in the drywall as they would have if it had been a wall. Take some spackle and fill in the various dents, holes, weird pockmarks in the ceiling. Then take a broom and sweep the now dry ceiling (the ceiling will be wet immediately after scraping, be careful at this stage as you can tear the drywall paper). There are going to be imperfections, try to fix them as best you can.
Step five: Say good-bye to your sanity. Unpainted drywall needs to be primed. It soaks up the primer paint like a dying man in the dessert sucks up water. We probably should have used two coats of primer, but left it at one. Its about this time that despair creeps upon you. Your arms ache, your neck hurts, and you realize that there are two more coats of paint left. Suck it up, the results are worth it, and really, you can’t quit now. Two coats of ceiling paint and you’re DONE!

Smooth as a baby’s bottom. (Don’t worry, that fan is being replaced.) We de-popcorned our whole basement (2 bedrooms, 1 hallway, 1 long family room). A whole trashcan was filled with the used spitball substance that was once “popcorn.” All the hard work is worth it, I promise, if you decide to do smooth out your ceiling.