Here's the problem: I've just bought my 1950's house and removed the wallpaper from the lounge. I've now got back to the plaster, but there's still paint left on the walls (in the pattern of the old wallpaper). I can scrape it off but it takes a lot of time and I was just constantly thinking that it might not be necessary, as there'll be quite a few layers of paint over it when it's finally finished. Will this short-cut work or will the end result just look rubbish and not be smooth? Another factor is that the person who was helping me before has painted a couple of the walls with this sealer for plaster without removing the paint first. Will I have to start again with those walls? Also, is the sealer necessary (it's quite expensive), as someone said that you can just whitewash them first. Also, someone else said that you can buy some sort of thick paint so that you don't have to remove all the other paint and it'll make it smooth. I don't mind hard work (as long as it's not too hard), but the cheaper option is probably best for me. Thanks loads for any help!
you could get a plasterer to skim them. not a cheap option but will give the best result.
from my experience that paint will show through. not the colour, but the outline. especially noticable when the sun/natural light hits it and casts shadows. any kind of uplighter/downlighter/spotlights with show them up too. your walls will look like the surface of the moon.
run your fingers around the edges of the paint that you are concerned about. if you can feel a ridge then it will most likely be visible.
a solution i've used with reasonable success is to get a wide scraper (or even better a taping knife - available any good diy store, just ask for one) and mix up some polyfilla, not too stiff, and skim around the edges of the painted areas with a wide scrim using the knife. feather out to nothing, a light sand and bob's yer mothers brother. its a good, cheap option - a fiver for a box of filler, tenner for the knife. and if you dont quite get it right first time just re-apply some more filler and feather it out a bit further. might need a couple or three applications to get it right.
consider paper lining afterwards, but note that lining paper on its own will not hide these types of imperfections (i speak from experience). those thick paints that you refer to work best if used with a textured roller and therefore give a textured finish, not everyone's cup of tea. its a bit of a corner cutter, cheap, quick and easy but not very satisfactory in my opinion.
with regard to sealing i would certainly consider this if you have gone back to bare plaster. the previous wallpaper paste may still provide a bit of a seal but is not guaranteed. i use watered down white paint (about 60-40 paint-water) and a dollop of pva glue in there helps a lot too. also gives a base coat for your colour. don't spend money on those expensive sealers, just a cheap pot of white matt paint will be fine. its only a base and youre gonna water it down anyway. shouldnt cost more than a tenner. another advantage is that it will give an idea of how it will look once the final paint goes on. anything undesirably still visible at this stage will show through after the final coat so it's a good opportunity for further prep work if needed without the need to strip the sealer (which you wont be able to do anyway if properly applied).
with the wall thats already sealed, just do as i say above with filler and taping knife. the filler will stick to the sealer no problem, then just re-seal over the top to seal the sanding dust.
Once I have stripped and filled a wall I lightly run an orbital sander with 120grit over it to remove any slight lumps, bumps or remaining fragments of paint or paper. It takes just a few minutes to complete a room but gives that touch smooth surface you need to give a good finish.
If the wall still isnt touch smooth then a lining paper provides an easy solution. The poorer the walls, the heavier the paper you use. Its not difficult to put up (honestly) and with just a little care butting the edges you will not be able to tell the room has been lined once its painted.
If the walls really are in poor condition with numerous large bumps and lumps that can not be removed without causing more damage then skimming is the only other option.