Painting on uneven walls

Help - we have just bought a 1975 house - we have stripped all of the wallpaper off, and underneath the walls seem fairly smooth - with the exception of a bit of yellow paint a few minor imperfections - How can I get these walls smooth without skimming?

I like painted walls, so do I need to spend hours sanding down each wall? or is there a quick fix way (I know its a stab in the dark! )
If I have to skim what sort of approx cost for 20' x 20' room?

Any advise would be appreciated.
Thanks

Fill the walls...sand down and line em with 800/1000 grade lining paper.

Thanks will get on with it at the weekend!

I like painted walls too

I find I get better results if I:

Use Finish plaster (not polyfilla) mixed fairly thin... like a thick custard. Just a bit of it in a margarine tub or something (this is assuming you're just perfecting the surface, not filling holes and chases which is a different thing).

Apply it with a broad filling knife (like, 100mm wide)

Apply hardly any, and use the knife to press it flat (no bulges that you will have to take off later)

After it's dry, scrape off any lines with a broad scraper (not sandpaper).

I give them a coat of thinned white matt emulsion when I think they're flat and smooth; that evens up the colour and suction and makes it easy to spot any defects so I can correct then before applying the colour. It also provides opacity to hide old colour mismatches or patterns on the wall.

(I am just a clumsy amateur)