What is the cement render mix ratio for external concrete block walls?
A good mix for rendering is 1 part lime 1 part cement and 6
parts sharp sand(1:1:6).
To ensure a good even mix the materials should be measured in
buckets rather than shovels, this will ensure consistency
throughout the job.
Water should be added in small quantities to the mix, the
consistency should be just stiff enough to stand on the trowel. The
area to be rendered should be brushed down with a stiff brush, then
doused with a hose pipe to remove the rest of the dust and prevent
the surface 'sucking' the moisture out of the render.
Always start at the top and work your way down and protect the
floor with with boards or sheets as it is quite a messy job. It is
advisable to apply in two coats, the first coat should be 'combed'
before it begins to set.This is done with a special rendering comb.
The second coat can be a little weaker, i.e 1:1:8. Dont worry about
any imperfections in the finish as these can be scratched off the
following day with a trowel to achieve a lovely even finish ready
for painting.
Its my opinion to use a ratio of 4 sharp washed sand to 1 part
cement, Bycol added to the water to aid in the flow of the cement
but its not that important once your skill level is up there. Lime
is not necessary and 1:6 is far too weak. 2 applications of render
should be avoided at all costs. No you cant scratch it off the next
day. Secrete/float the walls 1-2 hrs max after its applied. When
floating the walls make sure the surface is sprayed with a fine
mist to keep it moist. Small golf ball size blobs of render mix can
be thrown on to fill any low spots whilst floating. And use a
teflon float not a trowel.
Be sure the wall prior to rendering is paint free & hosed
down. the more water on the wall the better and if in direct
sunlight work in small sections...2x1 mt
