Plastering Outside Walls
Large areas of wall surface to be plastered should be divided into sections to make for systematic working. Use 10mm battens to divide the wall area into 1.52m sections.
Fix, with masonry, the first batten at a corner, ensuring a by checking with the builder’s level. The spaced battens will give working bays.
Start work at the bottom of the first bay. The mortar should be pushed on to the wall and spread, working upwards. Fill in each section and then level off the surface with a straight edge. Allow the mortar to set; this takes at least four hours. Then criss-cross keying marks, diagonally across the coat.
Cement paints or painted rough cast and shingle may be a little more difficult to work. If any material is loose, it should be removed, hacked out, and the bare patches made up to the level of the surface, with a 1:4:1/2 lime mix. Next, to give a good key, scratch the surface.
Smooth-surfaced stucco or cement work should be brushed down well with a wire brush and roughened to provide a key. A PVA slurry, consisting of one part PVA to one part sand and one part cement should be used.
A mortar plasticizer will help to spread the mixture. Lime added to the mix will facilitate the spreading power but increase suction. Suction is the rate at which the base coat absorbs water from the top coat. A high suction rate is required for some finish coats, while others need low suction.
Next apply two backing coats. The rendering coat consists of a 1:6:1 lime mix and is applied with a steel trowel. Mix these materials dry, turning well with a shovel. Make a hole in the middle of the materials and pour in water, push the inner part of the ring into the water and then add water again. Turn, until the mixture is well wetted, then mix thoroughly. If needed, a dry colourizer can be added at this stage.
Apply the mix with upward strokes, using the full length of the blade to ‘lay off. The coat should be smoothed out evenly with a floating rule and then roughened to provide a key. Comb the surface with diagonal strokes in two directions. This coat should be allowed to dry for 12 hours. The rendering coat should be 6mm thick.
The second coat, known as the floating coat, is applied to give a level surface. Lay a 275mm border round the area to be covered. This coat should be 12mm thick and is often best laid in two 6mm coats. Work each area systematically, a strip at a time, filling in the rendering as necessary and ruling off until there is an even covering. Ruling off reduces the thickness of the coat to 10mm.
The coat is then smoothed off with a wooden float. It depends on the finish coat whether or not the surface is roughened to give a key. If a roughened surface is required, use a comb to scratch horizontal lines across the surface.
A simple sandy-textured look with the appearance of natural stone is known as cement-fining.
Portland cement fining
This is applied to a surface with a low suction rate, so use only a little lime in the floating coat. Fining is applied to a keyed surface. The sand used should be sifted or even washed to remove any clay, which can cause the finished surface to craze or crack. Sand is nixed in proportion of two parts to one part of Portland cement. The cement can be white or grey, or coloured for decorative
Mix the sand and cement to a firm con-sistency and apply on to a 6mm floating coat, which should be ‘ironed’ into the base coat and then rubbed over with a wood float. If there are hollows they can be filled n at this stage. Use a sponge to finish off, with light, delicate circular movements.
The same finish is used when a stone block, or Ashlar effect, is required. Decide on the size of block needed and then draw a master line, using a spirit level, to make sure the work lines up.
Mark out the rest of the horizontal lines, working from the master line, then mark in the vertical lines. If any of the edges are not quite accurate or have become slightly blurred, they can be smoothed off with a sponge.
An Ashlar surface, well executed, looks like a surface of stone blocks. It can be white, grey or coloured with a colourizer.
A variation of Ashlar is known as vermiculation. This is textured effect to give variety to an Ashlar surface. It is quite a lengthy process and for this reason a random effect is suggested.
Using a sharp steel object, cut out irregular shapes in the block while it is still wet. Leave a border of at least 13mm round the block and 6mm runs between each cut away area of the design.
10. November 2011 by admin
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