Lime training II

Also did this one on limewash.

APPLICATION OF LIMEWASH

There are many different ideas about limewash mixes and whilst some people advocate a mix as thin as skimmed milk and in many coats (as much as twenty) I have seen mixes like thick cream, applied in a single coat that still work well. It seems that preparation and aftercare are more important. Though it is unlikely that this was often achieved traditionally it will minimise the risk of flaking or dusting.

If pigments are to be added these should be kept to a single colour as it is very hard to maintain consistent colour between batches.

The surface should be wetted and allowed to soak in so that the surface appears damp but there is no standing water. Apply the limewash with a natural bristle brush (use a cheap one because the limewash rots them very quickly) working it well into the surface to fill irregularities and cracks. A fresh coat will be translucent but as it carbonates and dries it will become more opaque. Limewash may dry at different rates because the water content varies within the substrate. Limewash sets by carbonation through absorbing carbon dioxide. If it dries too quickly it cannot carbonate and becomes dusty, however if it is too wet it will remain translucent. If it dries too rapidly it should be lightly wetted or covered with dampened hessian.

Limewashing should be avoided in very hot or cold weather. I suggest that the best time is at the end of summer and early autumn when the weather is cooler and damper.

Assuming that the existing surface is old limewash this should be brushed down to remove any loose areas and dust.

Old render that has never previously been limewashed should be inspected to check the cement content. If it absorbs water readily then it may be a relatively soft mix or even contain some lime. Limewash will not bond to a hard cement mix. This will require thorough cleaning to remove moss and lichen, especially if it is roughcast or harling. If one applies a thin limewash it will kill moss and lichen and it is possible to leave this for a few weeks, months or even a year before brushing it down and limewashing properly.

If there is modern masonry paint this should be scraped off or removed with a pressure washer or the Stonehealth DOFF system. However great care is needed as all pressure washers, including the DOFF can cause great damage if used carelessly. Limewash can be applied over small patches of paint. It will be patchy but this will help seal edges of the paint and as it is very alkaline it will accelerate its decay.

If further coats are required it is important that the previous coat is allowed to carbonate thoroughly before applying the next, otherwise the brush will lift the previous coat. The surface should be wetted again before applying the next coat.