I have steam-stripped 30 year old wallpaper from a number of rooms and am left with sticky walls. When I removed the wallpaper using a steamer in places the glue melted and was sticky like jam. How do I remove this in preparation for decorating? The walls were plastered 30 years ago before wallpapering - do i need to apply a primer once I've removed the wallpaper paste? Should i sand the walls (they are quite bumpy)?

Try Zinnser dif, its formulated to remove paste. You will have to go over the walls several times, the instructions are pretty clear.
You can also use sugar soap, whichever you use you'll need a scrubbing brush or similar and use plenty of elbow grease and when you think you've got it all off do it one more time.
Its a really horrible job but you will not get a good paint finish if any paste is left on.
If you do find a patch of paste it will liven up when the paint is applied to it, if this happens take a wide scraper and remove it.

If the walls have never had a paint finish you will need to apply a mist coat.
Normally we apply a non vinyl contract matt but as the plaster is 30 years old i think youd be safe with any matt emulsion you may have lying around, but consider white first as its easy to cover with finish coat.Before applying dilute the paint by 30%, once dry then sand the walls and brush the dust off with a soft broom, then fill, rub down, prime the filler the same way you did the wall, quick rub down and your ready for emulsion

Stripping down interior wallpaper requires the same meticulous preparations you did when you first installed it. When you're still using the traditional kind of wallpapers and wallpaper murals, you need specific materials and some chemicals also to make sure the removal will be successful and that the process will not damage your wall.

Also when stripping down wallpapers, it is advisable that you wear gloves to protect your hands.

~Creative Interior Design Ideas