Render on Blocks using Natural Hydraulic Lime (NHL)

To obtain the highest vapour permeability a block construction should be built with an NHL mortar.

Sands: always use well graded coarse sand (3 or 4mm to 0.075) unless indicated. NHL mortar will not alter the colour of the sand. See also Sands for Lime Mortars.

Main recommendations: make sure that the application surface is dampened before first coat (stipple coat) application. On smooth surfaces or surfaces with poor suction an SBR resin can be added to the first coat (max. 3% of the weight of the lime). Mix mortar well making sure that it is not over saturated with water.

Note: Volumetric mix ratios have been adjusted, when necessary, to the nearest round or practical figure.

Stipple coat (3-5mm MAX):
This coat should be cast on (or sprayed on) fairly evenly and left as cast to provide good keying. Scrape off high spots.
Dosage: 1: 1.5 using NHL 5

WAIT at least 2 DAYS. Dampen well before applying 2nd coat.

2nd coat
(10-15mm) Dosage: 1:2 using NHL 3.5, cast on or lay on.
Level with straight edge. After sufficient hardening, key with a devil float.
WAIT 5-7 days in accordance with performance related to weather conditions).

Note: In hot weather or windy conditions, dampen surface at least once a day with a fine spray (first 2-3 days). Protect from frost and adverse weather. See "Protecting Lime Mortar". Dampen before applying finishing coat.

Finishing coat:
A - Fine wooden float finishes - max 5mm thickness -use finer sands (1-2 mm-down) Dosage: 1:2.5
B - Coarse/Textured finishes - 6-8mm thickness with 3mm-down sands. Dosage: 1:2.5

Two coat work on concrete blocks is achievable. In this case using a spray gun is highly recommended.
Pre-mixed coloured EcoMortar is also suitable for rendering on blocks. See EcoMortar Data Sheet.

See also Rendering with a Spraygun and NHL Renders

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For further Guidance, contact your St Astier Distributor.
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