These two words get used interchangeably in conversation and mean quite different jobs on a quote. Knowing which you are being sold is the single most useful thing when comparing plastering prices.
Skimming
A skim is a thin finish coat, typically 2 to 3mm, of finish plaster applied over a surface that is already flat and sound.
The candidates are new plasterboard, existing plaster that is in good condition but tired or covered in old textured coating, and walls that have been made good after minor works. The plasterer is providing a smooth, paintable surface, not correcting the wall.
MyJobQuote puts skimming over sound plasterboard at £15 to £25 per square metre.
Plastering
Full plastering means a two-coat system. A backing or browning coat goes on first, often 8 to 11mm thick, to build the wall out and bring it to a level plane. A finish skim goes over that.
This is what bare brick needs, and what a wall needs when the existing plaster has been hacked off. The backing coat is doing structural levelling work that a 3mm skim physically cannot.
MyJobQuote gives £30 to £40 per square metre for a two-coat replaster, rising to £35 to £55 where walls are old or uneven.
Which one your wall needs
Tap it. Sound plaster gives a solid, dull knock. Plaster that has lost its key to the brick behind sounds hollow and drummy, and will often flex slightly under pressure.
Hollow plaster cannot be skimmed. Adding weight to a surface that is already detached brings the whole lot off, usually in the first year. It has to come off and be replaced.
Also look for movement, bulging, or plaster that crumbles when you press a screwdriver into it. Any of those mean replastering rather than skimming.
Why the distinction matters when comparing quotes
Two quotes £400 apart on the same room are frequently not competing at all. One has priced a skim and the other a hack-off and replaster.
The cheaper quote looks attractive until the plasterer arrives, taps the wall, and tells you it needs to come off, at which point the price changes. Or worse, they skim it anyway and you have a wall that will fail.
Always ask which job is being quoted, in writing, and ask what happens if the plaster turns out to be blown once work starts. A good plasterer will have already tapped the walls before quoting.
A third option: overboarding
Rather than hacking off, some plasterers will fix plasterboard over the existing surface and skim that. It is faster, far less messy, and avoids the waste disposal cost of removing old plaster.
The trade-offs are that it brings the wall forward by around 15mm, which affects skirting, architrave, sockets and window reveals, and it is not appropriate where the wall needs to breathe or where damp is present.
For ceilings, particularly old lath and plaster ones that have lost their key, overboarding is usually the right answer rather than a compromise.
Lime versus gypsum
In older South London terraces with solid walls and no damp-proof course, the original plaster is often lime. Lime is permeable and lets moisture move through the wall and evaporate.
Replacing it with gypsum and cement seals that route, and the moisture reappears elsewhere. On period solid-wall properties this is a real consideration rather than an upsell, and it is worth asking any plasterer what they intend to use.