Internal walls

Lime plasters were used up to the 1950s and are very common in Victorian and Edwardian buildings. Often when wallpapers are removed, significant amounts of plaster come off, requiring a great deal of preparation work to be undertaken. It was common in the early 1900s to cover blown plaster with heavy paper in an attempt to hold it together and disguise uneven finishes (hence 'papering over the cracks').

Blown plaster is the delamination of the plaster coats from the wall. When tapped, a hollow sound is heard. Blown plaster in itself is not a structural defect, and the concern is mainly restricted to the potential for it to fall off during redecoration, adding time and money. Lime plaster is no more prone to blowing than cement or gypsum plasters, as the main cause is workmanship-related. The main difference is that during application, lime plasters need long periods of time to set between coats. In the 1800s, 3 coats were required, each needing several weeks between coats, and constant wetting, to allow carbonation to take place. If coats dry out before they have set, they will not be strong enough and may de-bond.

The solid walls of the period can suffer from water penetration at any time during their life. When water passes through brickwork it can bring with it, or activate, salts or nitrates. Each of these has effects on the plaster that can cause it to lift from the wall. Each event is probably very small, but over time can equate to quite a large defect. Blown plaster may not be a detachment from the wall, but the detachment of layers from each other. When removing coverings it is better to be gentle so as only to disturb the layers, rather than the whole finish. The use of steam strippers can cause the plaster itself to be steamed off, and care should be taken not to hold the steam head on the wall for too long.

A lime plaster may have been on the wall for quite a considerable time. In that period it will almost certainly have moved, expanded, contracted and allowed various chemicals to evaporate through it. It can therefore be assumed that at some point, somewhere on the wall a layer will have become debonded as a normal consequence of age.

Repairs of blown lime plaster rely on the skill of the plasterer to prepare and apply the coats. It is quite common to find Victorian lime plaster walls with modern gypsum skim coats over them, and this does not appear to affect the performance of the wall. Unless the wall has major water penetration problems, the skimming with modern plasters can be fairly inconsequential.

Care needs to be taken with modern sealants used to prepare the walls, so that permeability is not impaired. Again in practice these coats are not precisely applied and are not known to cause major problems regarding the breathability in solid walls. PVA-based adhesives should be avoided as a bonding agent, as PVA emulsifies when wet and is ineffective if left to dry before applying the plaster. Simply wetting a wall can also result in unpredictable bond adhesion. Silicone-based adhesives are better suited in this role, and make for a more robust remedy (Unibond is a popular brand). If plastering over previously plastered bare brick, additional bonding preparation is needed, to ensure a good bond. Usually the application of a specifically formulated bonding agent is sufficient. It is preferable to use lime plaster on lime plaster walls, but time and cost can be a major influence, which usually means gypsum is preferred.

On lath and plaster walls and ceilings, the size of the gaps between the laths is important. De-bonding may occur if the gaps are too wide or too narrow, as this prevents a good initial key for the base coat.

Cracks in plasters can provide a wealth of information, and with careful observation can be evidence of minor irritations or early warnings of movement. See Viewing rooms.