Plaster finish
Also called: white plaster, marcite, pool plaster.
Plaster, also called white marcite, is the traditional interior finish of a concrete pool: a cement-and-marble-dust coat troweled onto the gunite shell. It is the smoothest finish when new, the cheapest, and the shortest-lived, at 7–12 years before it needs replastering.
Why it matters
Plaster is pure cement at the surface, so aggressive water etches it fastest. It is calcium-based cement that leaches into the water and pushes pH up, especially in its first months, so you add acid constantly. Hold the water balanced-to-slightly-positive, pH 7.6–7.8 with a neutral-to-positive LSI, or the finish etches and pits. New plaster is the fragile stage: if the water goes aggressive while it cures, it pulls calcium out of the surface and leaves permanent microscopic voids. Plaster also wears faster under salt, so a salt pool is better served by an aggregate finish.
In numbers
About 86 percent of the cement in a new plaster finish cures in the first 28 days, which is why wheeled and robotic cleaners stay out of a new plaster pool until after 28 days.