Exterior Thin-Coat Mineral Render, Self-Cleaning - Scraped Finish, 2 mm Thick

The process looks simple. The result, though, depends on the order of the steps, the waiting times between them and the weather. A renderer usually works like this:

1. Check the substrate. It has to be firm, dry, clean and free of dust. On an insulation system, the reinforced base coat with mesh must have cured properly, usually for several days according to the manufacturer.
2. Apply the primer that belongs to the chosen render. The primer evens out how much the surface absorbs and improves adhesion. Let it dry completely before rendering.
3. Prepare the material. Ready-mixed pastes only need stirring. A dry mineral mix is combined with an exact amount of water, the same every time, so the whole wall ends up the same shade.
4. Spread the render with a stainless steel trowel to the thickness of the grain.
5. While the material is still fresh, rub it with a plastic float in circular movements. This spreads the grains evenly and creates the typical fine, grainy "scraped" texture.
6. Finish each continuous area, for example one wall from corner to corner, without stopping. The work is done "wet on wet".
7. Protect the fresh render from rain, direct sun and wind, usually for at least one to two days.

Keep an eye on the weather. During application and drying, the render, the wall and the air should be roughly between +5 °C and +25 °C. The exact limits are in the manufacturer's technical data sheet. Overcast, still days suit the job well. On a south-facing wall in full sun the material dries so quickly that it cannot be floated properly.

A scraped 2 mm finish is made in several types. They look alike, but they behave differently.

• Mineral render is based on cement and lime. It lets water vapour through and does not burn, and its alkaline nature resists algae at first. It comes as a dry mix, usually white or in a limited range of colours, and is normally painted with a facade paint once it has cured
• Silicone render repels water very well. Rain runs off and takes dust with it, which is why it is often called self-cleaning and why the facade stays clean for longer. It is flexible and holds its colour well. It suits shaded walls, walls near trees and areas that get wetter than the rest
• Silicate render lets vapour through well and stands up to the weather. It is harder to work with because it sets quickly and needs an experienced hand
• Acrylic render is flexible and comes in many shades. It lets much less vapour through than the others, so it is usually not used on mineral wool insulation

On mineral wool insulation you choose a render that is open to vapour: mineral, silicate or silicone. On polystyrene boards all types can be used. It is sensible to stay with one insulation system from one manufacturer. The adhesive, base coat, primer and render are designed to work together. If you mix components from different brands, the manufacturer often will not give a warranty.

Several things affect how demanding the work is, and therefore what it costs:

• Condition of the substrate: a flat, freshly reinforced base coat is the simplest case. An old facade first has to be cleaned, cleared of algae, repaired where cracked and sometimes levelled
• Shape of the facade: many windows, corners, cornices and sills mean more edging and more joins
• Access and height: a taller house needs scaffolding and work at height
• Size of continuous areas: a large wall has to be finished without a break, so several people must work on it at once
• Weather and timing: work must stop in unsuitable weather, so the date can shift
• Colour: dark and strong shades are more demanding on the material and on even application

If the look of the facade changes or you are adding insulation, ask your local building authority what the local building regulations require. Do not assume in advance that nothing is needed. In a block of flats, the building management or the owners' association decides about the facade, not the individual flat owner.

Most faults are not visible straight away. They appear after the first winter or after a few years. In practice, these come up again and again:

• Visible joins and patches: work stopped in the middle of a wall, or the render was applied in full sun, and stripes remain once it has dried
• Floating too late: the material has already started to set, grains get torn out and the texture becomes uneven
• Rendering onto a damp or uncured base coat: this risks efflorescence, cracks and poor adhesion
• Skipping the primer: the wall draws water out of the render unevenly, which causes colour differences and weaker bonding
• Missing diagonal mesh at window corners: strictly a fault in the base coat, but the cracks appear in the render
• Too dark a colour on insulation: a dark surface heats up strongly in the sun and can damage the insulation. System manufacturers therefore set a minimum light reflectance value for the colour, often given as LRV or HBW. Always check it before choosing a dark shade

Look at the finished wall during the day in side light, when the sun shines across it at an angle. That is when unevenness and joins are easiest to spot. At handover, check that:

• The texture is even across the whole wall, with no bare spots and no clumps of grain
• There are no visible stripes, joins or patches of a different shade
• The render finishes neatly at windows, sills, the plinth and edge profiles
• There are no hairline cracks around windows or at corners
• Windows, sills and paving have been cleaned of render
• You have received details of the system used, the type of render and the batch number
• You know when and with what the facade may be painted, and how to look after it