I have freshly sprayed knockdown textured ceilings. I can not figure out if priming is necessary before coating it with flat white ceiling paint. Some people say it is okay to just paint it, some say it needs to be primed. Could someone clarify it for me?
Priming is almost always the correct answer under any top coat. The rare exception is freshening a recent (< 5yrs) top coat with a like formulation (oil to oil, latex to latex).
That said, my recommendation is to leave popcorn/texture UNcoated, if possible. Removing it AFTER painting is much more difficult.