Preparation: Prepare the wall by hacking off any loose render and scraping out any mortar joints to firm material. Make sure your wall to be rendered is really clean and etched so you get maximum keying and suction between render and masonry, Fix vertical battens to the wall about 10mm thick, about every 1.5 metres. Fix the first to a corner, making sure they are straight and upright even if the wall is not. Each space between these screeding battens will be filled separately. This is the way to keep the render flat and even. Put boards or a tarp along the bottom of the wall to collect droppings, they are inevitable. You can hose this off. You will also need to wet the wall down first before you start rendering to aid in suction (spray wall lightly to make damp, not dripping wet).
Render mix: Exposed position: 1 part cement, 1 part lime, 6 parts clean sharp fine sand. Sheltered position: 1 cement, 2 lime, 9 sand. Don't make the mix too strong in cement compared to the substrate or you could risk delamination and drummy render, add only the correct amount of cement. Always batch the materials don't shovel them, and mix them well. Mix only as much render as you can do in 1/2 and hour, (if you are like me that wont be much) and avoid stretching the mix by adding extra water, remember the cement is chemically reacting and going off in the barrow. Use clean dry plasters sand not brickies sand which has clays in it. Add the correct amount of clean water to get the best trowelable mix, too wet and it will slump and too dry and you wont be able to apply it (crumbly). If you use an additive like AV600 or even bondcrete make sure the that adding lime as a plastiser is ok with the manufacturer . If you just use lime in the mix you shouldn't need a Bycol additive to flow the mix, as the incorrect use of Bycol type products can affect the strength of mortars and renders. Dont be tempted by the old wives tales of adding dishwashing liquid detergent into the mix this is worse than incorrect use of Bycol. If you must use a thickener use something like Dynex.
Application: With a steel float spread the render onto the wall with an upwards motion from a small board held against the wall. Fill in between each batten until a bit higher than level. With a straight edge board, level off between screed battens with a see-saw movement. Fill in hollows. Renders are built up of two to three layers, first a rough keying scratch coat 5-10mm, a second body coat of 10-15mm with scratching and then a final finish coat 5-8mm, (the thickness can vary from render to render and some only use a two coat system). The reason for building the system up is to allow the wall face to be squared up and avoid lumping on great gops of render which are unlike to stick, (the render slumps) and have an uneven drying pattern and crack. In hot conditions outside you may need to control excessive drying, shrinkage and cracking by keeping the render moist with a sprayer (not dripping wet), this is fairly critical for good render.
Finishing: Finish with a wood float for a matt slightly rough finish (the usual) or a steel float for a smooth paintable finish. Alternatively you can apply a second thin skim coat of render to finish off if you want a really good job. In that case don't float the surface yet. If you want to apply a finish coat of render, allow to set for 3-4 hrs, and then lightly scratch the base coat in a criss cross pattern with a nail. Lever off the battens carefully, fill in the gaps, and allow 24 hrs to set. Then dampen the surface and apply a skim coat. Finish off as above. If no finish coat then lever out the battens after floating and fill in the gaps. Feather out the edges with the float. The finished job will be 10-12 mm thick.
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