07/04/2016
Sand is underrated. It provides the structure of plaster,
and the quality of your sand can make the difference
between success and failure. So what makes sand good or
bad? In general, good plaster sand should be sharp, with a
diversity of particle sizes, and clean.
Sand should be sharp and angular, not worn and
rounded. Imagine trying to build any kind of structure out
of balls vs blocks, and the reason for this becomes
obvious. Unfortunately this means that many natural
sands are poor plaster sand. Beach sand in particular
should be avoided, because waves have often been
rounding the sand grains for thousands of years.
Particle size diversity is important to create good
structure, and to reduce the amount of binder needed.
Imagine a bucket filled with softballs, how many golf balls
could you add to the bucket without changing the total
volume? Then how many marbles could you add to that?
Ideally you’d have a mix of nearly every grain size so that
there are few large voids left – this creates a structure
that resists movement, and also requires less binder to fill
all those voids. Less binder equates to less cracking, and
shrinkage cracks are one of the plasterer’s number one
enemies.
Sand should not, however, contain silt – which is the
particle size below sand, slightly coarser than clay. Silt fills
the voids in place of the binder, resulting in weak plasters.
Clay can also cause serious problems in lime-based
plasters. Salt also can lead to plaster failure, as well as
causing rusting of metal lath or any other metal used in
plaster preparation. So when we talk about sand being
clean, we mean free of fine particles, and unwanted salt,
chemicals or organic matter.
As a conservative rule the largest particles in your sand
should be no more than half the thickness of your plaster,
but preferably would be at least one quarter the thickness
of your plaster (larger aggregate can provide better
structure, resulting in a stronger plaster with less
cracking). So if your plaster coat is a half inch, your
largest aggregate would ideally be between 1/8 and 1/4
inch. There are several types of sand that are widely
available, so when you call a sand yard, or any
construction materials supplier, you need only tell them
what you want and it will promptly appear at your
jobsite… maybe. Unfortunately the definition of sand types
allows huge variability (even assuming it is followed
correctly), and what you receive on the jobsite will depend
on what that supplier carries, or what is locally available.
Nevertheless, as a rough guideline the main sand types
everyone carries are masonry sand, concrete sand, and (if
you’re lucky) stucco sand or plaster sand as it is known.
