you have to take the paint off, thats the best way, 98 percent is good enough, and if you are reskiming you can do it rough. I have a special tool for this work, looks like a spade but its a scraper, its muscular work and dusty so if you arent up for it I quite understand, find someone with a construction vacuume and nothing to do, good luck, and dont listen to the knowall who says you can just plaster over paint, Its very stupid, I know I did it., you could I suppose get a machine for that, what about a heat gun, and tou could also try wetting the wall, works on wall paper depending on the glue.but youd have to scratch it up a bit first, wet it up after the days scratching, what about a wood rasp, real big one, a saw held on the right angle can be good.