How to guide for Kirk on lime mortar and plaster

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Render Systems

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SPECIFICATION: External Hydraulic Lime Renders

Preparation: follow good working practices, ensuring that the background is thoroughly clean. If removing vegetable growth using biocides check that these would not react with the render causing stains. If the background is saturated check for cracks or other causes of water penetration, faulty gutters or drainpipes, cracks in the joints of terraced roofs.

Causes of damp should be remedied and cracks should be cleaned out, (packed with low fire clay tiles or slate if necessary) and sealed with an NHL pointing mortar and allowed to cure for 1 or 2 days before rendering starts.

Suction Control: if necessary, apply sufficient water to reduce excessive suction, especially on bricks and porous stone. On many occasions this is done the day before, if necessary several times with the last damping just before application starts. Apply water starting at the top of the structure.

Old bricks require more water than new ones. The top of the structure will dry out before the bottom. In base coats this means that scouring back and keying of the lower section might have to be done later than the upper section. Always dampen down before applying subsequent coats

Keying: Sometimes joints are raked back (normally 10mm), this is not always necessary with NHL renders unless the background is very smooth and not porous. Stipple or Spatter dash coats can provide adequate keying to the background. Criss-cross patterns using a pointed wooden lath are much preferred to combing when a key is being provided to the scratch coat.

Dubbing Out: on defaced surfaces or in areas with a large amount of damaged joints it might be necessary to apply a dubbing out coat to provide a relatively level surface. In most cases this will be sufficient and joints or holes will not have to be filled with rubble unless quite deep.

When a dubbing out coat is used let it set sufficiently (8-10 hours) before keying it. The most efficient way to apply a dubbing coat is by harling. The strength should, as always, be compatible with the type of background but a strong mix is recommended (2-3 or 1:2) once the correct NHL has been chosen. Apply base coat or scratch coat after approx. 2 days (more if very deep recesses have been filled) depending on weather conditions affecting drying out time.

Setting Properties of NHL Mortars: The setting properties of NHL mortars require much lesser time for protection against adverse weather conditions than fat lime mortars. Precautions are however necessary and for further advice contact us for further details. Protect from frost, heavy rain, strong wind or direct sun for a minimum of: -
NHL 5 : - 48 hours
NHL 3.5: - 72 hours
NHL 2 : - 96 hours

The preferred form of protection is mortar fleece or hessian cover which, which can be pre-dampened to contribute towards curing the mortar. Frost protection should be applied even if frost is not occurring at the moment of finishing the day's work but is forecasted during the night or within a short time (see suggested protection periods above) from completion of work. Work should not start in frost conditions or with temperatures below 5°C. When working with NHL 2 or in rendering with fine finishing coats, this should be 8°C. Protection from drying winds or direct sun should be provided by using shading sheets or debris netting on scaffolding.

Reworking: NHL mortars can be reworked (up to 24 hours with NHL 3.5 and NHL 2). This is due to the absence of cement or gypsum in the lime and due to the minimal quantity of Aluminates. Reworking diminishes potential waste, allows preparation of some mortar to be left overnight so that a quick start can be made in the morning. When preparing an NHL mortar for later use, place it on a board after mixing and cover it to avoid contact with possible rain or sun. When re-mixing add the least possible water. In some cases no water at all.

Rendering on Different Materials: Where different materials meet, and where there are timber lintels and other changes in the background material it is necessary to insert a metal mesh at the joint (at least 100 mm. each side). Consider the different suction characteristics of the background material Hair can be added to the first coat applied to metal lath to increase the bond and tensile strength.

Ensuring a Level Surface: to achieve a uniform and level surface fix vertical timber battens on the wall at 2-2.5 m. interval. If the wall is uneven use spacers and check that battens are straight with a plumb level. Screed off excess mortar between battens with a wooden straightedge spanning between the battens. When battens are taken down, fill in strips with the same mortar. An alternative is to make running screeds 10 cm. wide at regular intervals using the battens as described above and applying the float coat in between them.

Scour back and key with a devil float after initial setting. Check for shrinkage during the first 2 days and, if necessary, lightly dampen the relevant area, scour back and re-key. Do not apply a finishing coat for 3-7 days, until undercoat if adequately firm and any small amounts of shrinkage are complete.


The mix ratio and NHL format will depend upon the type of substrate, its relative hardness and condition also its exposure level on site. (Typically 5-2 sand - lime)

A Stipple or Spatter dash Coat (optional): can be used on strong, smooth and variable backgrounds. The normal thickness varies between 3 and 5 mm. This coat has to provide sufficient bonding to support the remaining coats of render. Use the maximum strength the background will allow (Flax Head Pike). The thickness of the first coat depends upon the nature of the background, the overall thickness required on the render.

Scratch Coat: (Millrigg Meadow, (blend of fine and course particles) to be applied 3 days (or more, depending on atmospheric conditions) after completion of the (optional) stipple coat. Thickness can vary according to the overall thickness required but it is normally between 10 and 15 mm. It must not be applied over 20 mm thick. If this is required it should be done as an extra coat (two intermediate coats) each not above 20 mm. The thicker the intermediate coats the longer the waiting time before each application. Key the mortar in readiness for subsequent coats.

Float Coat: (Elm Cragg, (blend of fine particles) to be applied 3-4 days (or more, depending on atmospheric conditions) after completion of the scratch coat. The thickness should be kept between 10 and 15 mm. Key the mortar in readiness for subsequent coats.

Finishing Coat: (blend of fine particles) to be applied 3-4 days (or more, depending on atmospheric conditions) after completion of the float coat. Use at 5mm approx for smooth or light textured finishes, 7-8 mm for coarse external finishes (Tyrolean, roughcast etc.) This can be applied in two thin coats immediately after each other.
Note: in all renders, coats should be applied firmly to exclude air and any excessive moisture. Suction needs to be carefully controlled at all times.

Finishing coats are available in a variety of fine natural mixes White Morn Scarr, Hawkstone Gold, Rose Cavern, Middlerigg Ruby, Mosswood Gold, Elm Cragg. No colouring is added.

Aggregates are described as sand or fine and grit or course in variety of particle sizes for sand or fine 2.36mm and below and grit or course 2.36mm and above that are needed to make the base for your lime mortar


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Render Systems said:
irish_spread said:
he wont be happy u stealing his thunder ::)

hahhah well i have put 3 vido guides up now on venetian plastering hope he likes them ;)

ur probably sh1ttin urself and hoping uve done everything correctly. Because he WILL
spot all ur mistakes ;)

kirk johnstone

Private Member
nice one render, and to the rest of you lot take note ;)
this is what this forum is for, helping each other. between all of us on here we could write a guide that covers absolutely every aspect of plastering in great detail :)
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