Coat Count and Thickness

How many coats plaster and joint compound need, how thick each runs, and the drying gap between them.

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 field, and the third finishes the surface.

Sanding sits between the coats and before priming: about 120 grit for flattening ridges and tool marks, and 220 grit for the final pass before primer. The fine pass removes the scratch pattern the coarse grit leaves behind.

A skim coat over drywall runs 1 to 2 millimetres thick and needs a PVA primer before the finish paint. Below about 1 millimetre, joints and fastener heads start to telegraph through the finished surface.

A gypsum finish over a base coat is a single layer at 2 to 3 millimetres, not a three-coat build. Applying the joint compound coat count to a gypsum finish, or the reverse, produces the wrong thickness and the wrong drying schedule.

Gypsum plaster sets in about 30 to 90 minutes after mixing, so batch size has to match what can be spread in that window. Over-batching leads to scraped-off material and uneven coat edges.

• Joint compound: 3 coats, about 24 hours drying between coats.
• Skim coat: 1 to 2 millimetres, PVA primer before paint.
• Gypsum finish: 2 to 3 millimetres over a base coat.
• Sanding: 120 grit to flatten, 220 grit before priming.
• Working time: 30 to 90 minutes after mixing.

Further reading

• Plaster Drying and Curing Times
• Plaster Mistakes That Show Through Paint