Joint compound | Chalkline

TL;DR

Setting compounds (chemical-cure, sold as 20, 45, 60, 90 minute working times) let a wall get three coats in a day. Drying compounds (premix or powder, water evaporation, around 24 hours per coat) are slower but more forgiving. The single biggest defect is painting over uncured compound: ghost joints and shrinkage cracks that no painter can hide. Match the system to AS/NZS 2589:2017 Level 4 for typical paint, Level 5 for critical light or gloss.

What it is

Cementitious paste that fills the recess between plasterboard sheets and embeds the joint tape, then builds up to a flat, sandable surface. Also called drywall mud, jointing cement, or just “mud” on site.

Properties

Form	Sold as	Cure mechanism	Typical pot life
Powder, setting (chemical-cure)	20 kg bag	Hydration (gypsum, plaster of Paris based)	20 to 90 minutes from mix
Powder, drying (vinyl-based)	12 to 20 kg bag	Water evaporation	Stable until used
Premix, drying (vinyl-based)	5 to 20 kg bucket or box	Water evaporation	Stable in sealed pail

Setting times for setting compounds are nominal (Cornice Cement 45 = 45 minute working life, 60 = 60 minute, 90 = 90 minute, per CSR Gyprock). Crushed set plaster accelerates cure; cold or wet conditions slow it.

Coverage (rule of thumb, manufacturer-confirmed before quoting)

Coverage varies by board layout and joint count, not gross floor area. Plan against linear metres of recessed joint plus penetrations and screw heads, then check the manufacturer’s data sheet for the specific product.

Grades and variants

Type	Where to use	Notes
Setting compound (hot mud)	Embedding coat or fast-track three-coat system on commercial-pace jobs	Mix only what you can use in the pot life. Not sandable to a feather edge in the same way as drying compound.
All-purpose drying compound (premix)	Embedding, fill, and topping coats on standard residential	Most common bucket on site. Easy to sand.
Topping (or finish) compound	Final skim coat only	Lower shrinkage, smoother feel under the trowel, easier to sand. Don’t use for embedding the tape.
Lightweight drying compound	All coats where weight and dust load matter (overhead work)	Lower density, slightly less hard.
Setting cornice cement	Bedding cornice, bulk fills, hot patching	Sets harder than drying compound. The 45/60/90 number is the working life in minutes.

Where to use

Recessed plasterboard joints (RE edge), embedding paper or fibreglass tape
Internal corners, taped with paper tape
Screw and nail heads (two or three coats for a level finish)
Bedding paper cornice, where setting cornice cement is the standard
Patching small dents, dings, holes after frame settling

Where NOT to use

Square edge (SE) joints behind tile or cornice. There is no recess for the tape; bridging cracks if filled.
External applications. Joint compound is internal-use only, paint or no paint.
As an adhesive for fixing plasterboard to studs. Use a stud adhesive (Knauf Premium Bond or equivalent), not jointing compound.
As a tile bedding or tile grout. Different products entirely.
On wet, frozen, or contaminated sheets. Adhesion fails.
As a structural fill at sheet-end butt joints with no back-blocking on a critical-light wall. The butt joint will telegraph. Either back-block, or accept a Level 5 skim across the area.

Three-coat system (AS/NZS 2589:2017 Level 4, typical residential)

The standard residential finish is a three-coat application over the tape. AS/NZS 2589:2017 sets out the application and finishing requirements for gypsum linings, with finish levels 1 to 5 (Level 4 typical paint-grade, Level 5 for critical light or gloss). The standard references ASTM C475/C475M for joint compound and joint tape specification.

Coat	Purpose	Typical width	Cure before next coat
Tape coat (embedding)	Bed paper or fibreglass tape into compound, fill the recess	100 to 150 mm	Full cure: 24 hours for drying compound, or per setting time + manufacturer guidance for setting compound
Fill coat (second)	Build up the joint flush to the board face	200 to 250 mm	Full cure between coats
Finish coat (third, “skim”)	Feather past the joint, leave sandable surface	300 mm or wider	Full cure before sanding

Sand only when fully cured. Premature sanding loads the abrasive with wet compound and tears the surface. Premature painting traps moisture and ghosts the joint shape through the paint film.

Tape choice

Tape	Best for	Avoid for
Paper tape	All flat joints, internal corners, ceilings, recessed-edge work	Self-adhesive applications (it isn’t); rough-handling repairs
Fibreglass mesh tape (self-adhesive)	Small flat-joint repairs, patching, low-movement areas	Internal corners, ceilings, recessed-edge ceilings (lacks tensile strength for movement zones)

Paper tape is stronger and the residential default. Fibreglass mesh is faster on patches but gives ground in tensile strength, especially at corners and ceiling joints (per industry tape comparison guidance, e.g. Westgyp).

Sanding

Hand sand with 150 to 220 grit on a sanding block or pole sander
Wet sand or use M-class shadow-vac sanders to keep dust controlled
Don’t over-sand the paper face. Exposed paper telegraphs through paint.
Don’t sand setting compound aggressively in the first hour after cure. It can grip an abrasive and chunk.

Tolerances and acceptance

Tolerances live with the system, not the material on its own. Refer to:

Plasterboard tolerances for sheet flatness, joint-ridge visibility, and surface finish requirements at PCI.
Plasterer tolerances for workmanship limits on embedding, set flatness under straightedge, and sanded-surface defect criteria.
AS 2589 finish levels for the Level 1 to Level 5 acceptance grid.

The compound itself is judged in the assembled joint, not in the bucket. The most relevant numerical claims (joint ridge band height, flatness under straightedge) come from the HIA Guide to Materials and Workmanship and the state Guide to Standards and Tolerances, and are tracked under HIA-001, HIA-002, HIA-012, and HIA-013 in the relevant article.

Working with other trades

Before applying

Sheets fixed straight, screws set just below the paper face, no breakthrough.
Site dry, warm enough for compound to cure (manufacturer minimum is typically around 10°C; check the product data sheet).
Penetrations cut and dust cleared from recesses.

During application

Dust extraction running on power sanders. Other trades cleared from the area or PPE-equipped.
Wet skips covered between coats so debris doesn’t drop into the next bucket.

After application

Painter checks set surface under the room’s actual lighting before priming. Critical-light walls and ceilings (large windows, gloss paint, downlight wash) need a Level 5 skim, not Level 4. Specify the level in the contract or scope.
Tiler does not use joint compound as a tile bed. Different substrate, different bonding behaviour.

See first fix / second fix sequence for the full lining-and-finishing choreography.

Health and safety

Respirable crystalline silica (RCS): some joint compounds contain crystalline silica as a filler; some do not. Check the SDS for the specific product. Sanding any compound generates fine dust regardless. The Australian workplace exposure standard for RCS is 0.05 mg/m³ as an 8-hour TWA airborne concentration (Safe Work Australia).
Gypsum and lime dust: irritant to eyes, skin, and airways. P2 mask minimum during sanding, P2 plus eye protection during overhead sanding.
Engineering controls: M-class or H-class shadow-vac sanders, wet sanding, or local exhaust. PPE is the last line, not the first.
Skin contact: prolonged contact with wet compound is irritating. Gloves recommended for all-day work.
Manual handling: 20 kg bags and full pails. Standard lifting limits apply.

For full silica dust controls, see silica dust controls (planned). For state WHS frameworks, see your relevant state regulator.

Suppliers

CSR Gyprock: full range, including Less Mess Multi-Purpose, Easy-Flow (machine-tool grade), Pro-Repair 10 (quick-drying repairs), Rapid Set (WA-formulated quick-dry), and Cornice Cement 45/60/90 for cornice bedding.
Knauf Australia: All Purpose Premix, BaseCote 45/60/90 (setting compounds at the matching working times), FinalCote and LiteFinish (finishing compounds), RediBase (premix base).
Siniat (Etex): full range across base, finish, and all-purpose.
USG Boral: now part of the Knauf group.

Trade pickup at building merchants, plasterboard wholesalers (Wright Forbes, Westgyp, Madex Linings, others by region), Bunnings Trade for smaller jobs.

[Sponsor / preferred installer slot. ACCC disclosure required.]

What can go wrong

Ghost joints under paint: painted before compound fully cured. The cure-time write-down is the cheapest fix; the rip-and-redo if it shows is the expensive one.
Shrinkage cracks down the joint centreline: too thick a single coat, or insufficient cure between coats.
Bubbles or blisters in the tape coat: tape laid over too little compound, or air trapped under the tape. Cut, refill, retape.
Hairline cracks at door and window corners: stress concentration plus rigid lining. Control joints or back-blocking help.
Telegraphed butt joints on critical-light walls: standard residential butt joint with no back-blocking, lit by raking light. Plan back-blocking or specify Level 5.
Sanding through the paper face: over-aggressive sanding past the compound feather. The exposed paper picks up paint differently and shows.
Sander tracks at handover: power sanding without changing pads often enough, or finishing on a coarse grit.
Set surface failing under tile in wet areas: joint compound is not a wet-area tile substrate. Use cement sheet or certified tile-backer board per AS 3740.

References

AS/NZS 2589:2017, Gypsum linings, Application and finishing (Standards Australia, SAI Global preview) (verified 2026-05-05)
CSR Gyprock product range, including Cornice Cement 45/60/90 and the all-purpose / repair compound family (gyprock.com.au) (verified 2026-05-05)
Knauf Australia jointing compounds, including BaseCote, FinalCote, LiteFinish, RediBase (knaufapac.com.au) (verified 2026-05-05)
Safe Work Australia, Silica overview and exposure standard (safeworkaustralia.gov.au) (verified 2026-05-05)
Westgyp tape comparison overview (paper, fibreglass mesh, Fiba Fuse), industry context (westgyp.com.au) (verified 2026-05-05)