Conversion of a garage to workroom or living
space or renovation of old masonry houses often calls for placing or renewing
cement render over internal brick walls. The rendered surface may then be
painted or plastered. The wall must first be cleaned of all loose particles and
be well damped down.
 The major problem is adhesion of render to the
surface. To achieve this, the surface may be chipped or picked in a close random
pattern, especially if it is smooth concrete, or a spatter coat of equal parts
of cement and sand, made to a fairly wet mix, may be splattered on to give a
rough key.
Alternatively, a keying preparation such as
Bondcrete can give a quick easy solution. Applied to the damp surface as
directed, and included in the water of the render mix, it will ensure adhesion.
The usual render mix is one part cement, one part
lime and six parts sand. As the mix needs to be kept plastic, only a limited
amount should be mixed at a time. In any case, the beginner should experiment
with a trial quantity to get the knack of smooth application.
Take a quantity of render on a board to the
dampened wall or dry, Bondcrete surface and work from the floor at a corner over
a width of no more than a metre. Slide the render onto the wall with a steel
float smoothing as you go. The coat should be about 15 mm to 20 mm deep. With a
large steel trowel, smooth lightly with a sweeping motion as large areas are
laid up.
A useful way of dealing with a large area is to
apply vertical battens of uniform size (say 25 mm x 20 mm thickness) at 1 metre
intervals. (These may be held by concrete nails direct into the mortar joints).
Then render in panels, using a screed board between the battens to level the
surface before a final steel towelling. Remove battens when the render has set
off and fill the gaps.
A smooth finish for domestic use is achieved by
applying a putty plaster, about 3 mm thick, with a steel trowel over the
rendered surface, which must have at least two days to dry. In hot conditions,
keep the surface lightly damped to avoid cracking while setting.
The putty plaster mix consists of equal parts of
dehydrated lime and hard plaster, mixed with water to about the consistency of
putty. The mix is made up in small quantities for it sets quickly, and it is
best applied from the top down, with a steel trowel, after damping down the base
coat of render. After covering a third to a half of the vertical distance the
plaster should be brushed down with a brush soaked in water, then steel
trowelled smooth.
Plaster can be applied over old, sound cement
render, or to a scarred internal masonry wall as a preparation for painting.
|
