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Plaster on Blocks – Guide

External Plastering (Rendering) On Concrete Masonry Units

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

Sands: always use well graded coarse sand (#5 to #200) unless indicated. NHL mortar will not alter the color of the sand. See also Sands for NHL Mortar.

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.

Scratch Coat (3/16″ MAX):
This coat should be cast on (or sprayed on) fairly evenly and left as cast to provide good keying. Scrape off high spots.
Mixing Ratio: 1: 1.5 using NHL 5
WAIT at least 2 DAYS.
Dampen well before applying 2nd coat. (3/8″ to 1/2″)
2nd coat
(3/8″ to 1/2″)
Mixing Ratio: 1:2 using NHL 3.5, troweled 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 for min. 72 hours. Dampen before applying finishing  coat.

Finishing coat:
AFine wooden float finishesmax. 3/16″ thickness -use finer sands (#12-down) Mixing Ratio: 1:2.5
B – Coarse/Textured finishes – 3/16″ – 5/16″ thickness with #6 sands. Mixing Ratio: 1:2.5
Two coat work on concrete blocks is achievable. In this case using a spray gun is highly recommended.
See also Rendering with a spray gun and NHL renders general guidelines.

 

All above information is given as general indication only. Dosages may vary in accordance to materials and aggregates used and site conditions. Please liaise with your St. Astier distributor to finalize mortars mixes. Good building practice is assumed at all times.

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