What it is called, by market
One concept, four markets. Where a name means something else locally, the card says so rather than leaving you to find out on site.
United Kingdom
Same word, different thingskim
also skim coat, finish plaster, multi-finish
A thin gypsum plaster over the WHOLE surface, applied by a plasterer — not a joint treatment, and the quantity is an area rather than a length of joint.
Australia
Same word, different thingset
also skim, topping compound
Australian practice more often follows the jointed approach than the British full skim, so the word appears in both senses and which is meant depends on the specification.
United Statesyours
Same word, different thingjoint compound
also mud, taping compound, skim coat (level 5)
Normally applied only at the JOINTS and fixings. A full-surface coat exists as the highest finish level and is a specified extra rather than the default.
Canada
Same word, different thingjoint compound
also mud, drywall compound
As the United States, including the finish-level system — so "finished drywall" describes a level rather than a full-surface coat.
The governing standard, by market
United States
ASTM C840 / GA-214
Application of gypsum board and the levels of finish.
United Kingdom
BS 8481 / BS EN 13279
Design and application of plasterboard finishing, and gypsum binders.
Calculators for this
Each works in either measurement system, and the terminology on the page follows whichever market you have selected.
Frequently asked questions
- What is the difference between skimming and taping?
- How much of the wall gets covered, which changes the material, the trade and the quantity. British practice normally applies a thin gypsum plaster over the ENTIRE board surface — the skim — producing a continuous, dense, smooth finish that is painted directly. North American practice tapes and fills only the joints and the fixing heads, feathering the compound out and sanding it flush, leaving most of the paper face as the painted surface. The first is priced by area and done by a plasterer; the second is priced by joint length and done by a taper. Reading one specification with the other in mind gets both the labour and the material wrong.
- What are drywall finish levels?
- A North American system describing how much finishing has been done, from the board simply fixed with nothing applied, through progressively more taping and coats, up to a level where a thin skim covers the whole surface. The point of the system is that it makes the requirement explicit: the finish that suits a warehouse is not the finish that suits a wall in raking light, and the difference is a specified level rather than a matter of care. The highest level is effectively what a British skim produces by default, which is the clearest way to see that the two traditions bracket the same range of outcomes and simply put their default in a different place.
- Why does raking light expose one and not the other?
- Because a jointed finish has a shallow profile the full-surface coat does not. Filling a joint means building compound over the taper and feathering it out, which leaves a very slight rise and a change of texture where the compound meets the paper — invisible under diffuse light and clearly visible under light striking the wall at a shallow angle, which is what a window or a wall-washer does. A full skim removes the distinction by covering everything in the same material to the same density. That is why the higher finish levels exist and why they are specified for walls in critical lighting, and it is a common source of dispute when a specification does not name the level.