set fair
To provide a smooth, finished surface on plaster after roughing in.
set fair: smooth plaster finish after the rough coat dries
To set fair is to apply a final thin layer of plaster over a roughed-in coat, creating a smooth, uniform surface ready for decoration. After the base coat (scratch coat or float coat) has cured to proper hardness, the plasterer spreads a finishing coat, typically 3 to 6 millimetres thick, and works it with a trowel or darby until the surface is flat, even, and free of trowel marks.
The timing is critical. If the base coat is too wet, the finish coat will not bond properly and may slump or crack. If it is too dry, the finish coat will dry too fast and may powder or fail to level. Most plasterers set fair once the base coat has firmed but still retains enough moisture to accept a new layer. On internal work, this may be 24 to 48 hours; on external surfaces or in hot, dry conditions, longer.
Materials and technique
The finish coat is usually a different mix from the base. Interior work often uses gypsum-based plaster with a finer aggregate; external or wet areas may use lime-cement mixes with additives for durability and flexibility. The plasterer applies the material in thin, overlapping strokes, using a steel trowel to compress and smooth it. The goal is a surface flat enough to receive paint or wallpaper directly, with no visible trowel lines or hollows. On high-quality work, straightedges and spirit levels are used to check flatness as the coat is laid.
Defects in set fair work include trowel burns (shiny, hardened patches from over-working), crazing (fine cracks from too-rapid drying), and slack patches where the finish coat lacks proper suction to the base. Poor set fair cannot easily be remedied and usually means stripping and replastering. This is why the task is entrusted to skilled plasterers and why supervision on site is important.
The term "set fair" reflects both the hardening of the plaster (setting) and the creation of a fair, or level, surface. In broader usage, set fair has come to mean any final smoothing operation on plaster work, whether internal or external, though the exact timing, material, and technique vary by substrate, climate, and the standard of finish required.