Drying time and coat count
Gypsum plaster finishes run 2 to 3 millimetres over a base coat and need roughly one day per millimetre to visibly set. Full cure takes 2 to 4 weeks, and paint waits until residual moisture reads below about 5 percent on a moisture meter.
| Parameter | Value | What it governs |
|---|---|---|
| Gypsum finish thickness | 2–3 mm | Coat laid over the base coat |
| Surface dry rule | 1 day per 1 mm | Time to visible set |
| Full cure window | 2–4 weeks | Stable wall moisture level |
| Moisture limit before paint | below about 5% | Meter reading that clears the wall |
| Working time after mixing | 30–90 minutes | Batch use before stiffening |
| Joint compound coats | 3 coats | Bed, level, finish |
| Drying gap between coats | about 24 hours | Sanding and recoating window |
| Skim coat thickness | 1–2 mm | Leveling layer before PVA primer |
Surface dry runs about one day per millimetre of gypsum finish, but paint waits for full cure and a moisture reading below about 5 percent.
A gypsum finish of 2 to 3 millimetres over a base coat visibly sets in about two to three days at the one millimetre per day rule. That is surface drying only: the wall keeps releasing moisture for 2 to 4 weeks before it reaches a stable level.
Paint should not go on until residual moisture is below about 5 percent, confirmed with a moisture meter rather than by touch or appearance. A finish that looks dry can still hold enough water to blister or flash under a topcoat.
The gap between surface set and full cure is where most paint failures start. Rushing the window between day three and week two is the single most common cause of patchy drying marks.
Three coats of joint compound at roughly 24 hours apart, or a single 1 to 2 millimetre skim over drywall followed by PVA primer.
Drywall joint compound is applied in three coats with roughly 24 hours of drying between coats. The first beds the tape, the second levels, the third finishes; sanding runs from about 120 grit for flattening to 220 grit before priming.
A skim coat over drywall runs 1 to 2 millimetres thick and needs a PVA primer before the finish paint. Skipping the primer leaves the paint sitting on a chalky surface that drinks unevenly.
Gypsum plaster over a base coat is one finish layer at 2 to 3 millimetres, not a multi-coat build like joint compound. The coat count differs by system, and mixing the two rules is a common error.
Gypsum plaster sets in about 30 to 90 minutes after mixing, so each batch has to be used before it stiffens.
A mixed batch of gypsum plaster has a working window of about 30 to 90 minutes. Beyond that it stiffens on the hawk and trowel, and re-tempering with water weakens the set.
Batch size should match what can be laid in that window. Mixing more than the crew can spread leads to scraped-off material and uneven coat thickness at the edges of the work.
Temperature and humidity shift the window, but the practical rule is to mix small and finish the batch rather than stretch it.
Each card carries one parameter with its value and what it governs.
Working values for gypsum plaster and drywall joint compound as listed in the reference table above.