Plastering, Rendering and Surface Finishing
Core concepts
Plastering (internal) and rendering (external) protect the wall, provide a true surface and take the finish. Success depends on the background and the mix as much as on the trowel work.
Preparation of the background
Brush off dust, rake out joints to give a key, remove loose mortar, and chip off high spots. Porous masonry must be dampened so it does not pull water from the mortar, but not soaked. Fix galvanised or fibreglass mesh over junctions between different materials and over chasing. Where the wall is very uneven, dub out in layers and let each layer set.
Mixes and thicknesses
Typical mixes: 1:4 cement:sand for external render, 1:5 or 1:6 for internal plaster, or 1:1:6 cement:lime:sand for workability. Standard thickness is about 12 mm internal, applied in two coats where thicker: a 8-10 mm scratch coat keyed with a scratcher, then a 5-6 mm finishing coat. External render may be 15 to 20 mm in two coats. A mix that is too rich cracks; that is the most common cause of crazing.
Technique and finishing
Fix dots and screeds to establish the thickness, rule off with a straight edge, fill and darby to true the surface, then float. Wood float for a sand-faced finish, steel trowel for a hard smooth finish, sponge for texture. Cure the work by light damping for a few days, and avoid plastering in direct hot sun or strong wind, which dries it too fast.
Traps in questions
Plastering on a dusty or dry background, applying too thick a coat in one pass (it slumps and cracks), over-rich mixes, no curing, and skipping the key between coats.
Worked rule of thumb
For 100 m2 at 12 mm thick: wet mortar = 100 x 0.012 = 1.2 m3; add about one third for voids and shrinkage in the dry materials, giving roughly 1.6 m3 of dry sand-cement volume to batch.