It might have something to do with the kind of ceiling popcorn you have, the type of plaster you put on the ceiling, or the thickness of your layer of plaster. If you can spray water on your popcorn ceiling and then easily scrape it off, then you might want to do that instead of covering it over. My popcorn ceiling was a plaster that was hard and had been painted. No amount of water was going to soften it and let it be scraped off. I used lightweight plaster, which let me put a thin layer on. I always suggest that you try this in a closet or other space that is not noticeable first. I did my garage ceiling first, and then a small 3x3 foot bathroom area. I waited two weeks to see how it did before continuing.