The increasing popularity of Bagged and Painted finishes for brickwork has indicated a need for a specification sheet covering:-
- (a) Brick Selection
- (b) Bagging Mortar
- (c) Batching Ingredients
- (d) Additives
- (e) Application Techniques
The following specification will attempt to cover the main points and trade
practice necessary to achieve a satisfactory bagged and painted finish.
In most cases Common bricks are selected for bagged and painted jobs. Two things should be considered in relation to the bricks, they are :
(1 ) Surface finish
(2) Liability to Effloresce.
The bricks should be Wirecut to provide a key for the bagging and where
required to carry a thicker coat. A smooth face common will generally only
carry a thin bagging coat, in fact they are often just slurry washed and then
painted. The texture similar to sand grains on the brick faces visible under
the paint. No thickness to the coating.
To avoid problems occurring when the paint is applied the brick selected
should have a Liability to Emoresce of Nil or Slight. Bricks with a higher rating have been known to blow off the paint, requiring repaint or at least touch-up coats.
Bricks should be selected to allow variations to the bagging coat thickness, Wire-cut face preferred, and with a Nil or Slight Liability to Effloresce. The efflorescence figure can be obtained from PGH Technical Support.
NOTE Where a brick of Moderate Liability to Effloresce is used, the walls
must be fully dry before any paint is applied. Bricks with a higher liability are not recommended.
Depending on the thickness of bagging required the following mortar mix will meet most situations. The amount of water added is varied to produce slurry, a very wet mix, grout, slightly drier, bagging mortar, the consistency of bricklaying mortar.
- 4 parts clean sharp sand.
- 1 part Portland Cement, G.P.
- 0.5 part Hydrated Lime.
The ingredients must be accurately measured using buckets or boxes. Shovel batching is NOT sufficient.
This is especially relevant where a coloured bagging is to be used, and will not be painted. Unless ingredients are properly batched the risk of a colour variation is a real possibility.
Oxides can be added to bagging mixes to produce a range of colours. The maximum amount of oxide is 10% of cement volume or mass. 10% is considered the maximum, by the use of selected sands the quantity of oxide can be reduced.
The addition of Bondcrete or Cemstik is sometimes recommended, but in most situations will be un-necessary. Although the adhesion to the bricks may be increased, the movement of moisture from the brick to the air may be impeded. This may cause a problem if any soluble salts from the brick or mortar need to get to the surface. The bagging coat could be forced off the bricks by the salt crystals.
For bagging to give the desired appearance the brickwork should be carefully laid up. The joints, beds and perps, should be full and finished flush off the trowel. No gaps or voids should be present in any joint.
In some situations the desired finish is a "flat finish". This flat finish will allow all the bricks to be seen and the joints will be full flush finished.
This requires a dry bagging mix to be applied to the brickwork soon after laying and while the mortar joints are still wet.
The dry mix is of the same proportions as the laying mortar but no water is
added. The purpose in using dry mortar is that it will only stick to the wet mortar joints and not to the dry bricks.
A smooth face brick is often used for this type of finish and the wall is painted on completion.
A similar finish can be achieved using a "slurry mix", a very wet mix applied to a finished dry wall. The coating can be kept very thin, in fact to a wash if required.
To resist drying too early, the wall can be wet down before bagging. This also reduces suction and assists in spreading the slurry.
This is normally mixed to the consistency of plastering mortar. The bricklayer applying it will quickly determine the water content after the first mix, taking account of brick suction, application technique and weather conditions.
The bagging usually consists of a coat 2 to 4 mm in thickness, applied using either a Sponge, Hessian bag material or thick Paper sheets.
The texture of the coat results from the particular material used to spread the mix and the hand movements of the bricklayer applying the mix.
The bagging mix is carried in a bucket, the Sponge or piece of Hessian
"cupped" in the hand to lift the mortar from the bucket, the mortar then applied and smeared over the bricks.
To produce a consistent appearance, all bricklayers on the job should adopt a similar technique, all working in the same direction.
The bagging and brickwork should be allowed to fuly dry before painting.
This will allow any efflorescence which may form on the bagging coat to be
removed by the Painter by dry brushing.
Allowing the bagging to fully dry out will reduce the risk of blemishes being
caused to the finished painted surface.
Select the brick that will meet the bagging and /or painting requirements.
Smooth face for flush joint painted applications, Wire-cut for applied bagging coat 2 to 4mm and painted after drying.
Nil or Slight Liability to Effloresce.
Coloured bagging mixes must be accurately batched to avoid colour variations occurring in the walls. ( all mixes properly batched ).