Technical Advice Note for Bag Rub Finish
Bag rubbing is a technique where the pointing of masonry is conducted in a manner closer to rendering, that does not (have to) extinguish the ‘character’ of the wall.
The joints to the masonry are prepared the same as for pointing, cleaned out to an average depth of 25mm to form a key, this depth will vary according to the nature of the stonework, but should not be less than 20 mm. The joints are then thoroughly cleaned and washed out by use of a hose (fine spray nozzle only) or suitable pump up sprayer, this is preferably done the day before application, and again immediately before application with a fine mist spray. A single coat (following any preparation required for deeper pockets) of lime mortar is then applied fully into the joints. Once the mortar has cured sufficiently it should then be rubbed up using hessian sacking or some other type of suitable material in order to press it home. Then after the initial “set” has taken place it is rubbed over again. The rubbing action should be carried out in such a manner as to press it well back into the recessed joint closing up any initial shrinkage that has taken place, at this point it is often scoured up with a sponge or suitable medium to achieve the desired finish.
Upon Completion the work should be covered over with hessian or other suitable material in order to offer protection from the elements, this should be kept damp in warm weather. The finished coating should be further dampened down by a fine mist spray as necessary, to allow the ‘render coating’ to cure naturally. Rapid drying, either by wind or sun will often result in a failure and adequate protection is therefore a must.
Bag rubbing is a very simple and cost effective technique that is subsequently painted (not essential) with either a lime wash or a lime paint. The overall appearance of the stonework needn't be lost, resulting in a surface that has been unified and tidied up at a fraction of the cost of pointing or plastering, making it an extremely cost effective wall finish that was very often meant to be covered in any case. This particular approach does not require a high level of skill to execute but care and consideration will determine the outcome. The mortar should be mixed using a blend of 50/50 standard coarse and fine sand, mixed at the ratio of 2 : 5 hydraulic/non hydraulic lime to sand.
Material consumption will be in the region of approximately 50 square metres per tonne of mortar (for guidance only). This will obviously vary depending on how and what the wall has been built from and it’s overall condition.
For further information or advice please contact the Cornish Lime Company – 01208 79779