Lime plasters

Shelter coats and consolidation

The purpose of a shelter coat is to provide protection to surfaces that suffer unduly from the effects of erosion from wind, rain and pollutants or to protect and consolidate water damaged plaster. Generally they are used as a last resort in the knowledge that they offer the only solution, even a short-term one, to particularly problematic surfaces on historically significant buildings. The principle of “less is more” is particularly relevant in deciding when a shelter coat is appropriate.

An example of a candidate for shelter coat protection might be an early medieval statue:
– of significant historic and architectural merit;
– carved from soft stone;
– that has been badly weathered but where sufficient detail remains to distinguish features;
– and where replacement or significant repair may be inappropriate for various reasons.

The complete understanding of the decay mechanisms involved is vital. Inappropriate methods may lead to further deterioration and sometimes to irreversible damage.

Shelter coats should be sacrificial in nature, always being reversible. They usually consist of a feeble or non-hydraulic lime, with or without aggregates. In many respects they are similar to lime washes, and it could be argued that repeated lime washing of specific areas could be considered one way of applying and utilising shelter coats.

Where a feebly hydraulic lime such as Saint-Astier® NHL 2 is being used and particularly where an aggregate is considered necessary, they can be applied by trowel, spray or brush depending on circumstances. However they are usually applied as a series of thin coats of lime rich sand mixes, pigmented or not and often require several applications over a period of time.

Backgrounds (masonry – brick or stone, plaster or rendering)
The background should be clean and free of all pollutants and generally have loose or flaking materials removed before any consolidation work is attempted. It should also be checked for the presence of damaging salts and treated where necessary. This may involve applying a poultice to remove salts over a long period.

Some lime stones that are very friable and delicate may require several treatments with limewater or other methods of gentle consolidation. Shelter coats are often applied to porous backgrounds and considerable pre-wetting may be required.

For protection and exclusion of direct rainfall to repair areas is very important, see Protecting Lime Mortar.

In some circumstances, friable surfaces can be consolidated by several applications of limewater. Saint-Astier® NHL 2 are particularly appropriate for consolidating poor backgrounds. Upon completion of consolidation, the background should be treated as any other background to control suction.

Consolidation materials
Always check that limewater will be beneficial before application, in some rare circumstances limewater would be detrimental, reacting with the primary agents of decay in the host masonry or background. In the case of lime plasters or renders, limewater almost always has a beneficial effect.

Consolidation example: 

Example: Badly weathered soft friable lime stone sculpture >  Spraying NHL 2 mixed in a ratio of 1:8 with clean potable water, lightly on to a pre-dampened background repeatedly over a period of time can significantly improve the surface condition prior to the application of a shelter coat.

Shelter coats materials
NHL 2 and fine aggregates, usually ground marble, limestone, crushed chalk or fine silica sand, with or without pigments. Typical mixes could include all of these materials.

Mixing & application
Shelter coat consisting of 3 parts NHL 2:1 part fine marble dust : 6 parts clean drinking water mixed to a slurry and allowed to stand for 12 hours.

Immediately before application, remix the materials vigorously ensuring complete mixing and apply to pre-dampened background with a bristle brush in several coats (minimum 5 coats with 24 hours between coats).

Finish off with 3 coats of simple lime wash made with 1 part NHL 2:4 parts clean drinking water again leaving 24 hours between coats. Protect work at all times (see Protecting Lime Mortar).