Two and three coat work
for NHL renders
Two coat work is suitable for renders with an overall thickness of approx. 20 mm on surfaces that provide adequate suction and a good key. On surfaces offering poor suction and keying, it is recommended to use a stipple coat (3-4 mm thick) applied by casting on, harling or spraying. The main coat can be applied after sufficient hardening and finished as required. Alternatively use 3 coat work by applying a finishing coat.
Two coat work
1) Base coat
On two coat work, the base coat will be the thickest (10-15 mm , more if applied in 2 passes) with a binder: sand ratio of 1:1.5 or 1:2. Use mainly Saint-Astier® NHL 5 or Saint-Astier® NHL 3.5. This can be laid on or preferably cast/sprayed on. Scour back and key after initial setting. To ensure a flat and uniform surface see “Ensuring a level surface” under the Three coat work section.
Check for initial shrinkage. If found, dampen surface lightly with water and tighten back and re-key. Repeated shrinkage is usually a function of poor quality sands, poor suction control or rapid drying.
3) Smooth and light textured finishes
Use finer well graded sands, 1-2 mm down to 0.075mm. Add just enough water to obtain required workability. The more water is added, the higher the risk of shrinkage. When the mortar is firm enough, proceed to float up with a cross-grained wood float. This is the most important phase of the finishing work and should be done diligently as together with good curing and protection, it is vital in obtaining a good finish. See Protecting Lime Mortar.
4) Coarse finishes
Use coarser sands if thick (rustic) granular finishes are required. The thickness of the coat depends on the final finish required. Some of these finishes, (especially the ones requiring special skills such as cottage, scraped and travertine effects) can also be done by using the same type of sand as smooth and light textured (floated) finishes. In these and tooled renderings (patterned), if initial shrinkage takes place, lightly dampen the surface and re-float the area during the first day or two. Tooling is normally applied when the render is 5-7 days old.
5) Dry dashing
Throw the chosen aggregate onto soft mortar and leave exposed. To speed up the work a plasterer throwing the aggregate can follow the laying-on plasterer.
Curing by water mist over 3 to 4 days (and if necessary more than once a day) is essential when weather conditions would cause quick drying. See Protecting Lime Mortar.
Three coat work
1) Base coat
It has to provide sufficient bonding. Stipple or spatter dash can be used on all backgrounds, but especially on impervious and smooth background. Leave these coats rough to provide a key. Use richer mix (1:1.5 preferably). The normal thickness is between 3 and 5 mm. On soft or weak background, use 1:2 or 2:5. Successive coats must be weaker than this coat. The thickness of the first coat depends on the nature of the background and the overall thickness required of the render.
A laid on scratch coat can be used on old bricks or surfaces providing a good key (greater care is required in application to ensure good bonding with the background). It will be scoured back with a cross grained wood float and keyed (crisscross keying pattern preferred) once initial stiffening has taken place.
2) Second coat (straightening)
It should be applied 2 days (or more, depending on weather conditions) after completion of first coat. Its strength should be less than the first coat. Thickness will vary according to the overall thickness required but it is normally between 10 and 15 mm. It must not be over 20 mm thick. If this is required it should be done in successive coats each not exceeding 20 mm. The thicker the intermediate coats, the longer the waiting time before each subsequent application.
3) Ensuring a level surface
To achieve a uniform and level surface, fix vertical timber battens or dabs on the wall at 2-2.5 m intervals. If the wall is uneven, use spacers and check that battens are straight with a plumb level. Fill out to screeds, if necessary in layers. 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 100 mm wide at regular intervals.
Scour back and key as usual after initial setting. Check for shrinkage during the first 2 days and, if necessary, lightly dampen the relevant area, tighten back and re-key. In case of intermediate coats this would apply to each coat. Do not apply finishing coat until undercoat is adequately hardened.
4) Finishing coat and curing
As per 2 coat work.
The setting properties of NHL mortars require protection against adverse weather conditions. Precautions are necessary and, if in doubt, your Saint-Astier® distributors will be able to advise further.
Early exposure to rain will cause some moisture absorption in the first few millimetres of a fresh render. If frost occurs, there might be damage. The figures given above refer, therefore, to a render that has not been subject to water penetration in its early life.
The preferred form of protection is hessian covers that, with re-damping, will also contribute to curing the mortar. Hessian covers are essential to protect against frost. Plastic sheeting is effective against rain but should be kept clear of fresh work. If too tight it will generate condensation leading to unsightly staining. It will not protect against frost. Frost protection should be provided even if frost is not occurring at the moment of finishing the day’s work but is forecast during the early days of a mortar. Work should not start in frost conditions or when frost is forecast or with temperatures below 5°C. In working with NHL 2 or in rendering with fine finishing coats, this should be 8°C. Protection from the quick drying effects of wind or direct strong sun should be provided by using shading sheets on scaffolding. See Protecting Lime Mortar.
Good work practices
In this document we have already discussed items such as background preparation, suction control, detailing, keying, protection and curing. A good and durable result depends mainly on these factors, the correct mortar mix, sand, dosages and workmanship. One item not to be overlooked is scaffolding.
Where scaffolding is being used, make sure that the scaffolding has adequate clearance from the face of the wall to allow application, avoiding unsightly lift lines. Scaffolding should project past all areas to be rendered to allow for protection of the new work against direct rainfall. Generally scaffolding should be capable of carrying the protective screens necessary to shade the work and prevent rapid uncontrolled drying and any covers needed to protect against frost.