My house was built in 1971. After removing a think layer of popcorn, I found cardboard behind it. Does anyone know what that is? Is it safe to just paint over it?
JACK has the correct answer.
I'd just like to add that the reason why I scratched all these holes is that I didn't spray enough water onto the ceiling, so the bound between the popcorn and the ceiling was too tight for me to use a knife to separate them.
After learning my mistake, the rest of the ceiling got scraped off very smoothly.