When a skim is enough
If the old plaster is firmly stuck to the wall and the problem is just the surface (cracks, filler marks, damage after removing wallpaper), a skim coat gives you a new, smooth finish without the cost and mess of hacking off.
When it has to come off
Plaster that sounds hollow when tapped, crumbles, bulges, or has been damaged by damp needs removing back to the wall. Skimming over it just traps the problem.
After damp work
If a damp-proof course has been put in or a leak fixed, the old plaster is often salt-damaged and will keep marking. It usually needs replacing with a suitable plaster.
Boarding as an option
On very uneven or damaged walls, fixing plasterboard and skimming it can be quicker and flatter than re-plastering.
Not sure?
Send us photos. Tom can usually tell from a visit, by tapping the wall and checking how sound it is.