- Gradation of sand in the finish coat
Another secret shared by Don Miles- using several
grades of sand in the
finish coat. This not only dramatically increases
strength in the mortar, but
adds crack resistent properties.
One large reason for excessive cracking in the
finish coat is the sand is
too fine. Your western style stucco finishes are sold in
a variation of two
grits, such as 40-60, or 16-20, for example. These means
the ratio of
coarse sand to fine sand. Even though these finishes
look nice, they are
on the right track. But they still aren't good enough.
I met someone in Seattle that used to plaster
boats. The formula they
used for boats was 7 bags of 7 grits of sand, for 2 bags
He had to guarantee the boats would withstand 8,000 psi,
incredibly strong. Samples were taken to break them in a
tester. Sometimes the samples would break at 12,000 psi
To put this into perspective, the footers under your
house are probably
required tp withstand 2,500 psi.
Sand sold here in Northern Virginia for stucco finish,
comes from West Virginia.
It is crushed quartz, graded into 3 grades, no.1, no. 2,
and no. 3, number three
being the coarsest. A grade finer than number one is
available from the factory,
and is called silica powder.
The disadvantage of this sand is the sand tends to be
shaped in round balls,
instead of sharp jagged grains. The sharper and more
jagged grains are better
for retaining water in the mix through setting and
curing. Also, the sand is graded
as no. 3 is number 3 only-nothing finer and nothing
coarser. Seven grits is
What do we use ? No. 2 sandblasting sand. No. 2
sandblasting sand has a
coarse sand, like no. 3 quartz, but is graded
differently. No. 2 is coarse
and everything below, providing a uniform mixture of
everything from coarse
down to powder. Also, Sandblasting sand,which is also
crushed quartz, has
a nice jagged grain, which looks like Southern
California desert sand.
Believe it or not, this fact is one of the largest
improvements we have made to crack resistence. In other
uses of portland cement mortar such as mud base
floors or laying block, the strength of the mortar
can be more than doubled,
simply by using various grits of sand.