Hi Goalc
It sounds like you got a right problem there and you want to prepare the area as best you can as it will show through the finished wallpapering,
if I was you, and I do it all the time in my customers houses if they don't want me to plasterskim or texture the wall/ceiling is to buy a bag of texture powder, the cheapest that you can find and use this as a filler, the benifits are it bonds well to the surface if it is not laid on too thickly and the surface is sealed well and also sound, but when completely dry it sands down really easy
I don't know if you are familiar with a caulker, this is a flexible plastic but rigid filling/feathering tool that is available at most diy stores here in the uk! use one of these as your applying & filling tool
first of all, make sure that you take any loose plaster ect off the wall
this may now leave a fine dusty residue which is best washed with a damp cloth
when this is dry seal over the whole wall with a pva (unibond ect), depending on the porousity of the surface follow the instructions on the container, this prevents the filler from drying off too quickly thus preventing it from blowing/falling off
let this dry completely, now your walls are ready for the filling
put a third of warm water into a builders bucket (don't tell the builder that your using his bucket, he may be bigger than you)
add the texture powder, completely covering the water and mix this in, the idea is to have the finished mixed texture powder to resemble a thick but spreadable consistency, so that it does not run or fall off your scraper
lay this onto the areas that need filling, smooth as much as you can and also feather in the edges of the filler to the existing wall,
this will shrink back slightly when dry as does most waterbased plaster and fillers
but when it is dry to the touch, but not completly gone white, caulk/fill over again, but this time go over the area of filler wider than the first,
or, if you want to, put the first coat of filler on, let completely dry, sand the edges to blend into the existing wall, seal with a watery solution of pva (as for porous surfaces) then fill again
Using a sanding block is good to get an even finish, also don't forget that sanding down any filling material creates a bit of dust and it may be best to use a dust mask
When all this is completed and your happy it will become very porous when dry so it is best to use a watered down pva and give the whole area 2 coats, allow this to dry then you should be ready to decorate
Good luck
Regards
Dale