Making Mortars, Plasters and Renders

• The hydraulic lime should be mixed with sand using a gauging box or bucket for proportioning.
• The minimum amount of water should be added to the mix to produce a workable mortar. The mix should be drier than a cement-based mortar. A conventional concrete/cement/drum mixer can be used.
• The mortar should be mixed for approximately 15-20 minutes to ensure thorough mixing and maximum plasticity of mix.
• Excess use of water must be avoided, as this will considerably weaken the mortar.
• The hydraulic lime mortar must be used within 2 hours of mixing.

Using pointing mortars.
Using plasters/renders.

Pointing

Preparation

• Remove all cement based pointing and mortar.
• Areas of sound lime mortar which is flush with the wall face or weathered back 8mm or less from the wall face to be retained.
• Old lime mortar should be raked out square, avoiding damage to arrises, to a minimum depth of 12mm or twice the joint thickness whichever is the greater.
• All loose material to be flushed from the joint and the joints thoroughly dampened.

Application

• During application it is important to ensure firm compaction of the mix; mortar should be pressed well into the joint using appropriate sized pointing irons.
• Mortar to be initially applied flush to the wall face. When it is leather hard it should be cut back flat with metal spatulas/pointing spoons to the level of the worn arrises of the stone. The contours of the worn stone should be followed with the level of the mortar, which should generally be set back slightly from the wall face.
• After cutting back mortar, the surface should be stipple brushed using a soft bristle brush to remove loose mortar and reveal aggregate. Avoid brush strokes appearing in the mortar.
• The work should proceed from the top of the wall downwards.

Typical pointing mix
Hydraulic lime 1 part
Well graded sharp washed sand 2 parts
Well graded limestone aggregate 0.5 parts

Protection and Curing

• Do not use hydraulic lime mortar if the temperature is 5 degrees C or below. Under no circumstances should the mortar be allowed to freeze during the first 7 days.
• At a temperature of 10 degrees C the initial hardening process will take approximately two to three days to be achieved. During colder weather, this time will be longer, and in warmer weather, shorter.
• Protect mortar from frost, excessive sunlight and drying winds for up to 7 days.
• Keep pointing moist for 7 days by light spraying with water each day. The chemical set can only complete in the presence of water.

Plasters and Renders

Application

• If there are large voids in the wall these should be filled first to within not less than 10mm of the wall surface.
• Render to be applied at least two coats by traditional methods to a minimum finished thickness of 15mm and a maximum finished thickness of 25mm.
• During application, it is important to ensure firm compaction of the mix.
• During application it is important to have a moist surface to avoid rapid suction of water from the render.
• After damping the surface (on very porous walls several applications of water in the hour or two before render is applied may be required) render can be laid on using the normal technique. The first coat (scratch coat) will normally require two skins of around 6mm each in rapid succession. Within an hour or two the surface is then scratched over using a suitable comb.
• Once the first coat has hardened enough a similar technique is used for the second. The time interval between the two will vary depending on the temperature - as little as 24 hours in warm conditions but several days if cool. A fine spray of water should be applied beforehand. If a third coat is required proceed as above. If a fine surface is required for the last coat a finer sharp sand may be used and 'floated up' as the surface stiffens.
• Rendering should not be attempted at temperatures below 5 degrees C. Render must be used within two hours of mixing.

Typical render and plaster mixes and thicknesses

Dubbing out coat (6-19mm) (1/4-3/4")
Hydraulic lime 1.5 part
Well graded sharp washed sand 3 or 4 parts

1st coat (6-19mm) (1/4-3/4")
Hydraulic lime 1.5 part
Well graded sharp washed sand 3 or 4 parts

Top (2nd) coat (3-6mm) (1/8-1/4")
Hydraulic lime 1.5 part
Well graded medium-fine washed sand 1.5-2 parts

Protection and Curing

• The render must be kept moist for several days after application of the final coat. Water will tend to drain down from the top of the work and therefore particular attention must be given to spraying the upper sections. Remember water is essential for the hydraulic set.
• It is important while maintaining the moisture to prevent rapid drying from wind and bright sunlight. Covering the work with damp hessian is the best procedure.