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What holds this stuff
A clear understanding of what holds plaster or
stucco mortar together is important in why this
material works, and why it fails.
The three kinds of bond are:
An example of a mechanical key is mortar on metal
lath. Mortar squishes through the holes in the
lath and sets up, locking the mortar in place. The
mortar that squishes through is called a "key".
An example of mechanical keys is this old
wood lath and
plaster application. The mortar that squishes
through the gaps in the lath forms keys that
lock the mortar in place when it sets up.
Chemical bond is used in several ways, through
paint on bonding agents, or bonding admixtures. In
paint-on bonding agents, such as plaster weld, the
surface of the bonding agent partially dissolves
when wet plaster is applied.
The wet plaster mixes with this dissolved portion,
and hardens, forming a bond. This isn't voodoo,
like some may believe.
Another kind of chemical bond is formed using
bonding admixtures. These add adhesive properties
to the mortar, sticking the mortar on the
Suction refers to the
absorption of water. A example of a suction bond
is laying bricks. The bricks are porous and absorb
water rapidly. It is this suction that absorbs the
mortar into the brick, creating a permanent bond.
This suction also holds plaster together. Too much
suction, or not enough suction, are the two main
reasons that plaster, interior plaster or exterior
cement plaster (stucco) fails.
An example of a high suction
substrate is cement block, also known as
cinder block or CMU (concrete masonry units).
Blocks absorb water very rapidly, making it
desirable to wet the blocks before applying
mortar. The idea is to slow down the suction, so
the mortar is absorbed into the block before the
water is absorbed out of the mortar. The more
water that is sucked out of the mortar before it
sets, the weaker the bond.
This it what is referred
to as a high suction surface if you see this on a
plaster or a manual on stucco. The bag of plaster
suggests adding more sand to
the mix for a high suction surface. The idea is
the same as wetting the block. The additional sand
retains more water in the mix, allowing more
mortar to be absorbed, and less water.
Other examples of a high
suction substrate is a scratch coat on metal lath,
where the scratch coat has set and partially
dried. The scratch coat should be wet down
thoroughly before the brown coat, or double up
coat is applied.
The same goes for brick or other
surfaces before the mortar is
applied. The suction is what holds the mortar on,
but high suction can be
controlled by wetting the surface. or adding more
sand to the mix on the next coat.
Photo shows the brown coat wet with a
hose to slow down the suction
application of the finish coat. This assures a
good bond and extends the working life of the
Too much suction
will cause the wall to fail, or the mortar to pop
off. A couple of examples of too much suction are
old soft porous bricks, or an old stucco scratch
coat that has set up for way too long. These
surfaces seem like they can't be wet enough. How
we deal with these surfaces is by using a chemical
bond. On cement plaster (stucco) we use an acrylic
admixture in the mortar. We use acrylic
mixed half and half with water. The acrylic
retains the water in the mix to keep the water
from being absorbed too quickly, as well as forms
a chemical bond by adhesion. This works on other
exterior surfaces, or interior surfaces that
receive cement plaster.
Other examples : Old blocks that have had
the mortar stripped off. The pores are filled with
dry mortar that have too much suction, Other
examples of too much suction are gypsum block, old
scratch or brown coats whet the mortar has been
torn off for repair, and existing finish coats
(color coats). On interior plaster, we use paint
on bonding agents to kill the suction by sealing
the pores and creating a chemical bond.
Plaster Weld. Euco Weld, and USG plaster
bonder work well for this. I have had failure from
other brands, which seem too watery. These paint
on bonding agents should never be used for outside
stucco, because they will get wet and fail.
Portland won't bond to white Portland,
contrary to popular belief.
Readers have written to explain they have had
their house re color coated,
and the new color coat is flaking off, or falling
off. A popular method for
re-color coating a house is to sand blast the
color coat back off, at least most of the way
before applying the color. This also removes any
loose and spalling areas.
My sandblasting days are over. We use the
same 50-50 mix of acrylic admixture when re color
coating existing stucco. If the surface has a
rough texture or has nonuniform suction, such as a
wall that has patches,it may be desirable to apply
a bonding coat first.
A bonding coat consists of the same acrylic
mixed in grey cement mortar, or mixed with pure
Portland cement (with no sand), and applied with a
stiff brush or a whisk broom. This leaves the
surface rough and the pores open for suction to
support the finish coat.
A good example of a low suction
surface, or too little suction is
concrete, both cast in place or pre-cast. Another
example is old terra cotta tile.
The concrete does absorb water, but very little
and slowly. Concrete is porous, but not porous
enough for a good bond. Cement mortar
applied directly to concrete will fail. Sometimes
it won't fail immediately, but it generally starts
to fail within about six months. This is why
readers have written to me about their stucco
falling off, and why so many of these fake stone
applications fail on new houses. The small amount
of suction and few open pores will hold the mortar
but not good enough. We have torn off and re-done
fairly new stucco applications that were done this
way. Our solution is the same: acrylic admixtures.
There are some old fashioned methods that work
well for this, and some applications where there
is too much suction, but I'll cover these later in
this article. If you aren't too bored, please read
Our mix is the same, half and half acrylic
with water in the mix. We put this in the scratch
coat for a typical three coat, 3/4" application,
or if the the concrete is real straight, a thin
base coat that is roughed up using a float or a
before applying the finish, or color coat.
Scarifying, (or raking, or scratching) the
provides a mechanical key for the brown coat,
but increases the suction by opening the pores
and increasing the surface area.
This is a
homemade scarifier I made from an old
may be desirable for ceilings, but with caution.
For inside plaster on metal lath ceilings, suction
can be increased by reducing
the amount of sand in the mix. For example,
instead of 20 shovels of sand to
a bag, the amount of sand can be reduced to about
12 shovels of sand to a bag.
This allows the ceiling to be brown coated or
doubled up the next day. Less sand and more
plaster makes the mortar more porous and increases
the suction. The idea
is to be able to double the ceiling the next day,
instead of letting it set and dry
the normal two days. Once we did this on some
ceilings in Alexandria, and let them set all
weekend. The ceilings had so much suction, we had
to soak them with a hose, and work in small
sections because the mortar was taking up too
fast. Taking up is when the mortar becomes firm,
before the initial set. It is probably better to
just use a standard mix, and let it set up for two
Suction in the scratch coat can
be reduced by adding more sand to the brown coat,
but this also should be done with caution. Too
much sand can also weaken the mix. Poor mortar,
that is mortar with too much sand, is also
slippery, and a lot of the mortar goes too waste
from dropping it on the ground.
I have to comment on posts I
have seen in places like the wall and ceiling
forum and other places on the web. I have seen
posted that a good mix for stucco ceilings is 3:1,
that is about 30 shovels, or 6 buckets of sand for
a bag. Please don't believe this. I doubt
anyone who says this has ever put mortar on a
ceiling all day. Our standard mix is about 2:1.
Much more than this and most of the mortar will
end up on the ground.
Straw Bale: I have never done any straw
bale, and it isn't used around here. What I've
heard is that mud plaster is thrown (dashed) onto
the bales and allowed to dry for a week or so.
This provides suction for the brown coat. If this
is wrong, please let me know. There are other
methods of straw bale.
Adobe: I haven't done any adobe, and it
isn't used in Virginia, but it used heavily used
in third world countries. Stucco will fail before
long because the cement and sand won't bond to the
mud adobe. Lime is used for a link. Soupy lime is
dashed on the adobe block and allowed to harden
and dry. The lime is firm and very porous and
provides a good suction bond for stucco.
mortar to concrete: This also applies to
other low suction substrates.
I have written about this before, which you may
find here: Question2002
An old fashioned method I
learned from an old timer and I have used before
is using a Portland cement paste. This method was
used in plastering swimming pools and on concrete
years ago, and still works today. A paste in made
using pure Portland cement and water, to a
consistency it can be applied with a brush. First,
the concrete is wet by splashing with water. The
paste is then put on
with a brush, leaving the surface rough. This
paste will set up immediately, and start to dry
quickly. It is IMPORTANT to put the scratch coat
of mortar before this paste dries. This can be
done in two ways. The paste can be applied to a
small area at a time, say about 3 feet by three
feet, and then the mortar scratched on. Another
method is to have two people working, one painting
on the paste and the other scratching mortar on
right behind him. The mortar is the scarified or
raked. and allowed to set up and partially dry at
least overnight. There is now good suction to
support the brown coat.
This is also a good method if have seen being done
in Mexico, and have seen foreigners doing here.
First the concrete is wet down and rich mortar
(mortar with less sand) is splattered on the
surface, leaving the splatters as rough as
possible. After this sets firm and partially
dries, a scratch coat of mortar can be put on and
scarified (raked, notice I don't say scratched).
This scratch coat can be browned after it has set
up and dried. I have seen concrete soffits done
like this that were three stories high. Just think
that someone could be killed if a piece of this
stucco mortar fell off and hit them on the head.
These ceilings will not ever fail if this was done
using this method.
Peppering is also used for using old welded
wire or woven wire lath, where there is no paper
behind the lath to support the scratch coat until
it sets up. The holes on the lath are too big or
the wire can't hold the mortar on. First, the
metal lath is peppered by splashing soupy mortar
on. Better suction is created by using rich
mortar, or less sand. After this peppering sets
and partially dries, suction is created for better
support on the lath, and the lath can then be
filled in with a scratch coat. I have never pumped
mortar on a ceiling, but I have heard that
peppering is used before pumping the scratch coat
on a metal lath ceiling. This suction
prevents "drop outs" when the scratch coat is
I mentioned I would bring up plastering on old
bricks without using chemical bonders. There is no
real good method, except first soaking the bricks
at least twice before beginning. One person can
then splash water on the wall while another
immediately applies mortar. If this mortar starts
to dry before it sets, more mortar should be
troweled over the drying mortar before it dries.
The idea is prevent the mortar from drying before
it sets, avoiding "dry-outs". These "dry outs"
will leave a soft powdery surface unsuitable for
applying more mortar. It is good to use a lot of
sand in the scratch coat for old bricks (meaning
1800's bricks). , say as much as 30 shovels per 94
pound bag of Portland. The additional sand retains
water in the mix longer to counteract the suction.
No matter what, on old soft bricks, the scratch
coat will have tons of road map shrinkage cracks
due to the high amount of suction. These will fill
in with the brown coat and should be of no concern
unless the edges of the cracks curl up, indicating
failure to bond. There is nothing else to do in
that case but tear it off and start over.
More about suction some other time. Suction is
also used in the brown and finish coats to provide
firmness and and workability.
I want to devote a following article to chemical bonding.
chemicals has improved the quality of our product
dramatically, but should
be done with caution. Please check back