One coat lime renders

working practices

Background preparation and types
The application surfaces must be clean. No dust, traces of other materials, paints, salts, organic growth etc… should be present.

Lightweight blocks should be brushed and, in the case of smooth blocks, keyed. Concrete surfaces should be brushed and washed. Traces of materials such as building mortar should be scraped off if they are over 1/3 of the proposed render thickness.

Dubbing out: in order to maintain the same thickness of render (max. 20 mm in one pass), some areas of the surface might need dubbing out with a suitable mortar at least 48 hours before the application of the render. In dubbed out areas deeper than 3 cm, a reinforcing mesh should be introduced. The render can be applied 4 to 7 days after, depending on the thickness of the dubbed areas, the weather conditions and the type of binder in the render mortar.

On cement surfaces the recommended pre-dressing Saint-Astier® mortars are:

1) Ready-mixed
– The same one coat render mortar, manually applied

2) Site-mixed
– Mortars made with Hourdex® (1:2.5 binder/sand ratio)

Wetting/Dampening: as a rule the background should be damp but dry on the surface. Dampening must take into account the overall background suction and the climatic conditions at the time of application and should be done about half hour before application.

Products with between 80 and 90% water retention must be applied on a correctly dampened background.

Products with higher water retention should be dampened in hot weather conditions, in the presence of strong drying winds and on high suction backgrounds.

Do not render on over saturated surfaces (i.e. if water runs off it after wetting) or on previously waterproofed surfaces unless these have been keyed properly.

On lightweight blocks it is advisable to apply a rough dash coat first to diminish the suction.

This is not necessary with DesignSprayable (Thermocromex®) on dampened lightweight blocks.

On very smooth and no-suction surfaces:
– apply a first coat with added SBR, or do any of the following
– key the surface with a chisel
– sand the surface to create a key
– apply a rough dash coat with added SBR

New masonry units should not be rendered for at least 1 month after production to be sure that these units have dried out and that most of their shrinkage has taken place.

On pre-cast concrete units or on poured concrete walls, the surface should be free of any demoulding oil. If not, this must be done by sanding or high pressure washing (min. 400 bars).

When rendering on masonry units with unfilled vertical joints, joints over 5 mm width should be pointed.

To avoid shadows when rendering on masonry units with large joints:
– either apply the one coat render in multiple passes leaving a day maximum between passes or
– apply a base coat and render on it after about 5 days

Always respect masonry joints if present.

Always respect manufacturer recommendations on water addition and mixing times.

Do not add any other material.

Always use whole bags.

Always use the same water dosage for each mix (proper measuring devices should be used).

Use the mixed material within 1 hour.

Do not try and re-work the mortar.

Protection and curing
Protect fresh work against adverse weather conditions. To avoid blooming, do not apply one coat renders with strong colours at temperatures below 8°C and high humidity. In hot weather or windy conditions, cure the work repeatedly with light water mist for at least 2-3 days.

Waterproofing effect
This is achieved when the render thickness is at least 10 mm. Do not apply in coats over 20 mm. If a thicker coat is required, apply in 2 or more passes at 48 hours interval. In cold weather this interval should be 1 week. The maximum overall thickness should not be over 40 mm.

Do not apply in coats over 20 mm. If a thicker coat is required, apply in 2 or more passes at 48 hours interval. In cold weather this interval should be 1 week. The maximum overall thickness should not be over 40 mm.

Possible finishes

1) Rustic (rough cast), partly smoothed rough cast and dragged finishes
Work in 2 passes with an interval of a few hours. The first pass has to be at least 10 mm to ensure it is waterproof. If the second pass cannot be done within a few hours, dampen the surface before its application. In sprayed renders, make sure that the pressure and the distance of the spraying nozzle are constant.

– In rough cast finishes the render is left as cast.

– Smoothing a rough cast finish is done once the render is slightly hardened

– Dragged finishes are achieved by scraping the surface once sufficiently hardened.

2) Scratched finishes
The render is applied in 2 passes: the first will be cast on or sprayed as a bonding coat (3-5 mm), the second is applied soon after. The total thickness of the work should be minimum 15 mm.

Rule the second coat immediately after application.

Scratching should be uniform. Use the appropriate scratching tools (devil floats etc…). It can be done between 3 to 24 hours after ruling, depending on weather the conditions

Remember, the finish (for example scratching vs. smooth) will impact the perception of the colour even if the product is the same.

Deep scratched surfaces are more sensible to rapid drying and therefore more attention should be paid to curing with light water mist. This should not be done in full sun or on sun baked surfaces (cure in advance and later on in the day).

3) Sanded finish
Proceed as per scratched finish. When the surface is hard enough, usually 48 hours but could be longer, sand it down after light dampening.

4) Sponged finish
Can be applied to various types of work, using different sponge texture or hardness when the work is sufficiently dried.

5) Smooth floated finishes (floats, sponge, trowel)
The render is applied to an overall maximum of 12 mm, ruled and floated when sufficiently dry. The minimum thickness must be 10 mm after floating.

Note: on large surfaces without joints it might be necessary to introduce joints. In all cases, if joints are present in the background, these should not be bridged.

6) Dry dashing
Apply the render in 2 passes to achieve a 15 mm overall thickness. Dry dashing is possible on the second pass when still fresh.

7) Lined finishes
Lined finish, simulating stonework, is achievable with the appropriate tools on a 15 mm render when sufficiently hard.

Visual appearance diagnostics
Normally not influenceing stability and durability

CRACKING not due to movement

Cracking webs: very fine, normally on surface only

Micro cracking: less than 0.2mm, various directions

Macro cracking: up to 2mm

Fissures : over 2mm
Wrong choice of product’s characteristics and specification with poor product knowledge.

Movement cracks in the background. Cracked joints. Bridged joints Wrong water addition.

Insufficient mixing time. Non application of metal or fibre mesh when needed.

High shrinkage due to background suction or climatic conditions.

Render too thick in places. Overworking the surface.
Render below 10mm minimum thickness.
Too deep scratches, reducing minimum thickness at lowest point.
Background not cleaned and prepared properly. Over saturated background High suction in the background Absence of a bonding coat on particular backgrounds Rapid drying of the render due to suction or climatic conditions especially in thin coats.
SHEARING Mainly on low strength backgrounds such as light weight blocks.
Wrong product choice.Background not dampened. Masonry units not ready (own shrinkage not completed). Absence of bonding coat.

This document is meant to give specifiers some guidelines and does not constitute a specification.