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How to Lime Plaster onto Laths

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Introduction

Lath and plaster is a building technique used mainly for interior walls in period buildings. A lath is a narrow strip of wood approximately 2 inches wide which is nailed horizontally to each stud in the frame. Each lath is spaced approximately 1/4inch ( thickness of the average little finger ) away from the next leaving enough space for the mortar to push through and hook over the lath to form a key laths.

Preparation

The LATHS must be free from dirt, grease and vegetation. These elements should be removed before damaged laths are repaired. Spray a clean cold water mist on to the laths prior to applying the scratch coat.

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First Coat

Using a ratio of 1 part LIME PUTTY to 2 and a half or 3 parts SHARP WASHED SAND with evenly distributed HORSE HAIR throughout the mortar and working to a layer of 10mm thick, push the plaster into and across the pre wet laths at a 45 degree angle to the laths.The plaster should be left until set hard but protected from heat and draughts.

Second Coat

The second coat also known as the intermediate or float coat will be applied similar to the first coat only without any hair being added to the mortar. You must ensure you wet down the first coat with a fine mist before spreading a 7mm coat, before scratching up and leaving in preparation of the finish coat.

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Finish Coat

For the final top coat use a fine mix of 1 part LIME PUTTY to 2 parts well graded super FINE SAND. Remembering to wet down the surface before application with a fine mist spray. Allow time for the water to absorb into the plaster then apply a coat of approximately 5mm. Rule off and leave for a couple of hours. When ready rub up the finish coat using a plastic or wooden trowel to bring back the fat until the required finish has been reached.  In the case of any light crazing this can be rubbed up and trowelled out.

Health and Safety

Lime mortar by its nature of application carries a risk factor, and therefore personnel should wear protective equipment, particular attention to be given to eye and skin protection. EYEWASH should always be on hand. GLOVES should be worn when working with any mortars or plasters.