As mentioned earlier, one of the most dangerous hidden drawbacks of popcorn ceilings is that some contain asbestos, a serious health hazard. If you have any idea that your ceiling contains asbestos, do not proceed with this DIY task until you have the material tested in a laboratory. Once you've cleared everything up, it's time to purchase some joint compound, also known as drywall mud. Lowe's sells his 3.5 quart bucket of Sheetrock brand multipurpose drywall joint compound for $11.98. An 8-ounce container of Red Devil's Lightweight Spackling costs $2.98 on Amazon. If your ceiling is heavily textured, consider 3M's high-strength, large-hole repair, fiber-reinforced spackling compound. The 12-ounce bucket is available at Ace Hardware for $9.99. It's easiest to paint, so make sure what you buy is a white or off-white shade.
How much drywall you need to buy depends on the size of your ceiling. You'll need to apply multiple layers to hide the popcorn texture, so expect to use the same amount of compound as you need to create the knockdown ceiling finish. So, use about 1 gallon of compound for every 50 square feet. If you would like a more accurate measurement, please use our online drywall calculator. I like what Dan the Drywall Man has to offer. If you go to the hardware store, grab a few sheets of 150-grit to 220-grit sandpaper (and your sander and sanding pole if you don't already have one), or some sanding blocks of the same grit.