Paint prep soap. Used to come as a powder that looks like sugar.

If the old paint is still attached firmly you can sometimes just put an undercoat with the sealer/primer you'd use to prep bare plaster. Did you do the test where you score an x and then see if the old paint pulls up with tape?

If you are putting acrylic paint over oil based paint you often need an undercoat anyway.

I've only ever gotten a good result on flaky/cracked/shitty paint by stripping and starting again. Took me two attempts on a big ceiling to learn my lesson.

The solar panels on my roof were part of this feat. It's a real nice feeling.