Mixing Render

Mixing plaster and render

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A basic lime render or plaster can be made using the same ingredients and mixed in the same way as a pointing mortar. A coarse sand can still give a smooth finish suitable for most vernacular buildings although the mix should be slightly richer in lime than a standard pointing mix, say one part of lime to two-and-a-half or three parts of sand.

Do not be tempted to use a soft, fine sand – you will just end up with lot of cracks. The mix will need to be slightly wetter than for pointing but it should be slightly stiffer than its modern cement or gypsum counterpart.

Once you start plastering you will soon discover whether your mix is the right consistency: too thick and it will be virtually impossible to apply it smoothly and get it to stick to the wall; too thin and it will go on beautifully then slump, sag or drop off.

Whether you mix your own or buy ready mixed lime render or plaster it is a good idea to ensure that the lime putty used is at least three months old. This will ensure that the lime is thoroughly slaked. If the lime is younger than this any unslaked particles in the mix may slake some time after plastering causing a small eruption or ‘lime blow’. Many practitioners advocate the use of six month old putty for plastering, but few suppliers stock it.

When plastering onto lath it is essential to add hair to the mix to help in forming a key of plaster between the laths. Whilst it is not essential to use a haired mix for other backgrounds it can help to reduce shrinkage. Hair should be added to the mix just before use. Do not add hair to coarse stuff that is to be stored for more than about six weeks as the hair may rot if left in damp lime mortar for a long time.

The hair should be gradually teased into the mix so that it is well distributed and does not form clumps (often referred to as ‘dead mice’). As a rule add 4 Kg of hair per cubic metre of mortar for work on laths. For the second coat halve the quantity of hair.

To check whether there is sufficient hair in the mix, scoop a dollop of mix into a gauging trowel, tap the underside of the trowel smartly against a hard edge so that the blob flattens and the surplus mortar falls off the edge of the trowel. There should be a fringe of hair at 1-2mm intervals around the edge of the trowel.