How do I get rid of "knock down" plaster?

I am in your exact situation . I can only imagine skim coat the entire ceiling.

It looks like it has been painted over so you can't likely scrape it off. There's nothing you can do that isn't going to involve work. You could either skim coat the whole ceiling or put up drywall to level it out and start over again. 1/4" drywall is the thinnest you could buy and would suit this purpose. When my husband drywalled our ceiling, he bought a drywall lift which made the job easy. He then sold it on Craigslist and was able to recoup most of the expense. He saved a ton of money doing it himself and then reselling the equipment!

I WAS in this same situation but my nestor neighbor happens to be a drywall contractor and he and his men are as much artisans as spanish spreaking mirical workers! they washed the surface with a light solution of dawn dish soap and water and scratched up the surface with a tool they called a wallpaper scuff. it is just kind of a round palm sized do-hicky with a few little steel wheels under it that run at odd angles and cut into the surface exposing the underlying plaster. it doesn't pull the old pain, just kind of cuts it up a bit. then as that is drying, they mix up a light solution of drywall mud. the box said it was a "light" weight mud? well, they added about a cup of water to it, maybe a bit more mixed it well. once done, the wall still not dry, they used large, very fuzzy rollers to skim coat the wall, and a VERY deft hand to trowel it smooth. the effect is a very clean very flat surface. they primerd the next morning and painted in the afternoon. they saved me SOO much work i had no idea this could be done effectively! Good luck folks! this is not a technique for the for sure, but it works and the results speak for themselves.

many hours of sanding