Lime plaster: Expert advice

Historic homes expert Marianne Suhr offers her advice on lime plaster.

Marianne's checklist: Authentic lime plastering

• Many plasterers will tell you they can use lime plaster, but in fact very few actually can. If applied by someone who doesn’t know what they are doing, it will almost certainly fail or crack. If you’re not sure whether your plasterer is up to the job, ask him what mix he intends to use. Internal lime plaster is usually based on ‘lime putty’, in a ratio of 1 part putty to 3 parts sharp sand. Hair is also an essential ingredient to prevent cracking. If he isn't able to recite this mix, then be suspicious.

• Lime plaster is usually applied in two or three coats, depending on how flat and perfect you want the final coat to be. The first coat should be ‘scratched’ to form a key for the next coat, then allowed to dry before any further plaster is applied.

• Lime plaster applied to brick or stone will dry quickly due to the ‘suction’ in the backing material. Plaster applied to a lath or reed mat ceiling requires considerable skill and strength. It will then take a minimum of two weeks before it is ready to receive a second coat. Make sure you plan this into your schedule.

• For a really smooth finish, a ’setting coat’ is applied. This is a mix of one part fine silver sand to one part lime putty. It requires experience and talent to apply this in a very thin layer.

• All these mixes can either be prepared on site or bought ready mixed from a lime supplier. They can be purchased in 25kg bags – or for larger jobs, they can be delivered to site in one tonne bags.

• Always ask your local lime supplier for advice. They may also be able to recommend lime plasterers in your area.

Painting on to lime plaster
If you are going to the trouble of applying a lime plaster, then it is important to follow through with a breathable paint. Traditionally, either limewash or soft distemper was used internally. They are available ready mixed or can be mixed from scratch using trusted recipes.

For homeowners who want a less laborious solution, there are various eco paints available. Before buying, check that such paints are breathable and compatible with lime plaster. I recommend clay paints by Earthborn (01928 734171; earthbornpaints.co.uk), which are particularly easy to apply and often only require one coat.

Read all about lime plastering a converted barn...

WORDS & PHOTOGRAPHS MARIANNE SUHR
Featured in the July 2011 issue of Period Living