Ensure a smooth plastered surface

By GEORGE OSOM

One of the cardinal considerations in building structures and masonry in general is how to set a smooth, sound and moisture-free plastered surface.

This guarantees the substrate (surface) resistance from fluctuating weather patterns, besides ensuring that the subsequent coats of paint or textured finishes adhere to the surface for longer periods.

Plasters are building materials used for coating walls and ceilings at the finishing stage and are manufactured as dry powders, which are mixed with water to form a paste.

There is a wide range of plasters — from gypsum plasters to lime plasters and most commonly, cement plasters.

Plastering of substrates, if well done, renders a sound, workable surface, which gives face to a structure. However, if the masonry goes wrong at this prime stage, the walls will be susceptible to a bevy of plaster-defects.

A poorly plastered wall can exhibit a cocktail of defects, which include blistering, cracking, flaking, unevenness and efflorescence. Most of these defects are as a result of poor workmanship at the masonry phase.

A blistered surface will have small patches of plaster swelling out due to late slaking of lime particles in the plaster.

CRACKING PLASTER

Cracking of plasters can be testimony to an imperfect preparation of the background or expansion and contraction (thermal variations) as a result of a thick plaster paste.

While poor masonry is to blame for unevenness of the surface, application materials are also possible culprits for this defect. Perhaps the most widely known plaster defect is ‘efflorescence.’ This is a crystalline white deposit on surfaces after the masonry work is done. It appears as a white chalky substance at the base of structures. 

Effervescence is as a result of the water-soluble salts present in the plaster. The salts are dissolved by the water trapped in the structure or that, which rises from the ground due to capillarity.

The saline solution evaporates and crystallises, leaving the chalky white substance.

This defect is common in regions of high humidity (coastal regions) and quite prevalent after a rainy season when the trapped moisture evaporates, leaving the white deposit.

Overall, when on a plastering job, the choice of the type of mortar for plaster depends on the atmospheric conditions, durability required, the desired finish and the geographical location.

— The writer is a painter trainer