Restoration, rendering and plastering tips.

Renders & Plasters 

External:- Historic buildings include a wide range of aggregate, binders and reinforcement over different backgrounds.  These include low strength daubs applied to wattle, low to medium strength renderings based on lime, high strength renderings based on hydraulic limes/cements.

Surveys:- Both visual and sophisticated analysis can be necessary in order to determine original material.

A conservation approach to old wall coverings is influenced by different criteria to those governing normal repair and maintenance.  The most economic approach may be to strip the whole wall and start again.  This may be the case for failing historic coverings.  However, a balanced view must be taken based on the significance and historic value of the surviving material. 

Failures:- These can be grouped into categories that may relate to each other.  Both symptoms and causes must be remedied.

• Loss of bond with wall – inadequate key / preparation / wetting, freezing, undercoat to strong for background.

• Loss of bond between coats – inadequate key / mineral bond, rapid loss of moisture to undercoat, swelling of clay materials in unwashed sand, outer coat stronger, inappropriate paint and finish used.

• Surface cracking – normal drying shrinkage, high strength plaster on a weak background, clay contaminated sand, structural movement.

• Friable powdery surfaces – rapid drying, migration of salts (these usually follow a pattern of moisture movement through the building)

• Surface spalling – freezing.

• New work – inadequate inspection / schedule of works, inappropriate use of materials, lack of protection during works, inappropriate paints.

Renovation work

Weak external plasters / renders tend to fail where there have been excessive moisture penetration and movements of soluble salts.

• Renders and joints to be removed and wall surface cleaned and wetted.

• Sacrificial coats may be applied to observe drying patterns.

• Hanks of hair to be included in backing coats. 

• Where different background materials occur these must be bridged.

• First coat must be applied to a pre-wetted surface and scratched for a key..

• Apply second coat before first dries, where necessary rub in water using a churn brush.  Cure slowly using a fine mist sprayer and protect from drying winds and direct sunlight with hessian.

• Avoid day work joints by cutting back previous days work by 50mm.

For increasingly exposed areas hydraulic limes would have been used in the past and should be used in restoration work.

Internal plasters

Pre 1930 ceilings usually comprised of three coats of plaster on riven or sawn laths.
Undercoats based on lime contained hair, sand and or pozzolanic material.
Finalcoats consisted of lime putty with fine silver sand or gauged with gypsum plaster.

Failures:-

• The repair of damaged plasters with harder modern product often results in cracking.  This makes matching original materials important.

• Sagging ceilings are often a result of ageing defects to structural supports, i.e. ceiling joists; and not necessarily as a result of the breakdown of the plaster.

• Long continuous cracks often occur along main supporting beams where counter-battening has not taken place.

• Shrinking joists exert tortional stress on the underlying plaster.

• Discoloration due to water penetration.

• Serious problems can be associated with decay in the ends of main structural beams.

• Fine wandering cracks are not necessarily problematic unless they widen over time or one edge is lower than the other.

Restoration work

Ceilings:-

• Where sagging plaster is to be retained the ceiling is supported from below using bearers with padded ply sheeting.

• Dust and debris is removed from between the joists using a soft brush and a vacuum cleaner.

• PVA and dampen surface.

• A key is formed on each joist using wire or mesh and filled with plaster.

• Enrichments can be further supported using wire binders fastened to the woodwork above.

• Washers and screws can be placed into the plaster and screwed into the joist above, the holes are then plugged with plaster and hidden from view.

Walls:-

• Cut out defective plaster at an angle so that any patches form a plug that cannot fall away from the wall.

• Clean and dampen walls.

• Apply 2 haired backing coats with a lath scratcher for trhe first key and adevil float for the second.

• Allow each coat to almost cure.

• Do not overfill so that the patch appears to be a bulge.

• Apply lime putty / silver sand finish cutting back around the edges so that no discernable ridges are felt under the fingertips.

Lime washes

A rule of thumb for the application of lime wash is that for every 5mm of new lime plaster 1 week of drying time is required.  Drying out time is important.

• New applications of lime wash paint can be adversely affected by residues in some paint strippers, e.g. sodium hydroxide in caustic soda.

• Old lime washed renders or stonework should be brushed and treated with an ammonium fungicide.

• External lime washes may contain oils or fats that act as waterproofers.

• Apply thin coats of lime wash up to 4 times.

• Premixed pigmented lime washes can be bought or they can be mixed using powdered pigments.