Patching traditional lime or gauged lime plaster is easy until you come to the final coat, so here are the most important things you need to know
But if it is, fix it well and to a high standard
if it isn’t broken – don’t fix it, sounds simple, but many people can’t recognise when something isn’t broken, or how broken it is and therefore how much repair is required. So here is a simplified guide and a short video clip taken from the Master stroke Series DVD about plastering with Lime this shows how to form a patch on lath using gauged lime plaster.
Patching Plaster – The Master stroke series xxxxxxxxxx
Plaster with lime falls into two main categories,
Raw Lime Plaster is lime and sand – it is slow setting, and may require 2 weeks between coats.
It is normally the case that raw lime and sand is three coat work – three distinct layers of plaster – irrespective of the background to which it applied a base coat – to adhere it to the background, and which requires any initial shrinkage to be tightened back and worked out of the slow setting plaster , a straightening coat, to make it dead straight – again any innate shrinkage is worked out by closing down the cracks and re-working the plaster with a wood float and a finish, in three coat work applied very thin on the straightened layer, troweled to a finish as it hardens by suction to the straightening coat. at anything more than 3mm thick is likely to crack, so the straightening coat must be as the name suggests – straight.
Gauged Lime is Raw Lime with a gauging of something to make it set faster
It is also important to recognise that gauged plaster is often 2 coat work to a finish. This means a base coat, which is the base coat and straightening in one operation and a finish, the finish is often considerably thicker than Raw Lime Plaster Finish – almost always 3-5 or even 5-8mm thick
Occasionally Gauged work is three coat, but this is the exception not the rule, if it is, it is generally not Gypsum gaged because – Gypsum swells a little (expands)and in so doing, it overcomes any initial shrinkage that occurs in Raw Lime Plaster and can be used in a single base and straightening coat, mortars that don’t shrink form exceptional good bonds quickly with the background – another identifier is that gauged plaster is generally stronger than – Raw Lime Plaster.
A hard strong three coat plaster, it is most likely to be Raw Lime Plaster with an Hydraulic Lime or cement gauging. Although providing a faster set, speeding up application time, it is not expansive and therefore the first coat needs to be applied as though it were a raw lime plaster, and any shrinkage cracking is worked out by tightening in with a wood float. As in Raw Lime Plaster the first coat bonds to the background, the second coat is the straightening coat and the final coat is a thin finish. A gauged cement or Hydraulic Lime finish, may well be 3-5mm but seldom more and can be Raw Lime Plaster as a finish coat, it often is.
Raw Lime Plaster is the original manufactured plaster in the UK after clay and dung plasters. Gypsum has been used in the UK for about 1200 years and the worshipful company of Plaisterers in London was established in 1501 for “Men who work with Gypsum and Lime” So Raw Lime and Gypsum plasters have been around a long time.
You might wonder why anyone would use a Raw Lime Plaster – especially since it takes so long o set – why not gauge everything or use fast setting plasters all the time – The answer lies in the use of the plaster – if you use a Raw Lime Plaster, you have ample time to get work perfect, you can lay it on one day and still work it the next, so the massive explosion in decorative plaster on Lath and on the hard – solid backgrounds, meant walls needed to be “PERFECT” so the ornate cornices and complex moulded skirting, picture rails and chair rails coal all be mitred in at corners and angles, perfectly square and perfectly true.
Grounds for skirting and picture rails, plaster screeds for cornices and staff beads for corners all became rules and guides for perfectly flat dead straight walls. If you have ever tried to make a large area perfectly straight with a plaster that sets quickly, you will struggle, it simply does not allow you sufficient time – unless you have lots and lots of highly skilled well organised craftspeople all working in harmony with each other.
Its so much easier to just stick with something thats forgiving, allows great dead straight flat or to be done more or less at leisure and with the manpower and resources you have.