When I did my lounge and hall stairs landing, i stripped the paper so I was left with the ‘snots’ (the blobs of old paint left in patches)
In order to get back to smooth plaster, i then wash the wall well to get the old paint really wet, and paint the wall with a vynal paint. I give the vynal paint a while to get touch dry, but no longer and then attack the not-quite-dry paint with a scraper. Lo and behold the scraper takes you back to bare plaster! (The new paint sscrapes off in sheets taking the old paint with it in the blink of an eye)
Be warned, my plasterwork is in good nick, and if you have carpets make sure you protect them well from semi-dry paint!