We want to have the surface of the exterior walls have a stucco appearance. After
investigating all sorts of alternative approaches (synthetic stucco, one coat stucco, etc), we
decided to use a traditional 3-coat system for its durability and cost-effectiveness. The only
problem is that we've never done it, and there is quite some knowledge and experience
required to do it correctly. There is lots of contradictory information on the internet, but most
sources pointed to the following system:
1. Metal lath securely fastened to the substrate. In our case we'll use one or two layers of
wire mesh screwed every 12 inches to the metal furring strips of the foam wall panels. I was
thinking of using two layers of mesh perpendicular to each other to assure a good grip of the
stucco to the lath so that the inner layer forces the outer layer to stand off the foam panel, but
some tests I ran later proved that was unnecessary..
2. A first layer (called the "scratch coat") comprised of (by volume) one part portland cement,
one part hydrated lime, and three parts sharp sand (relatively coarse). Many sources
recommended the inclusion of some sort of fibers as a binder in this layer to reduce cracking.
I found out that although fiberglass is often used for this, the alkalai in the cement literally
destroys fiberglass within about three weeks, making it functional only during application.
Nylon or polypropylene holds up indefinitely and provides long term integrity. A company
called Nycon (www.nycon.com) sells nylon fibers for many concrete applications in both small
and large quantities, so I bought a few packets for my tests (see below). I applied a scratch
coat layer both with and without the nylon fibers. The fibers made the stucco easier to apply
and gave no shrinkage cracks. The portion without the fibers showed some cracks wherever
the underlying lath was not lapped well.
The scratch coat gets raked horizontally with a serrated trowel before it sets too firmly to
provide a mechanical bond for the next layer.
3. A second layer (called the "brown coat") comprised of one part portland cement, one part
hydrated lime, and four parts sharp sand. This layer goes on as smoothly as the coarse sand
will allow, which isn't very smooth although it can be troweled flat. I played around with
applying a section, letting it cure for a couple of days, and then applying an adjacent section. I
wanted to see what the effect on cracks and appearance of the joints would be. Cracks don't
seem to be a problem (yet, anyway), but I found it useful to feather both edges of the joints (at
different times, of course) with a wire brush to virtually eliminate any visible lap.
4. A finish layer (called the "finish layer", surprisingly enough) comprised of one part
portland cement, one part hydrated lime, and three parts #30 silica sand (very fine).
Most sources called out the use of rods and screeds to achieve flat layers, but I found that the
sand grains pretty much set the layer thickness if you let them, and the only leveling required
is if the underlying wall surface is uneven. Our foam panels are holding pretty true, so I don't
think that will be an issue.
So I built a 4ft x 8ft artificial wall using 2x4's and plywood, covered it with lath, and started
trying to teach myself the art of stucco work. The results can be seen in the picture below.
Most of the panel now has the finish layer on it, which we'll use to test various methods of
colorizing. I left all three stucco layers visible in this section of the panel for illustration and
also to see what happens to each with time and weathering.
We originally (and naively) planned to stucco the house ourselves,
but common sense eventually overcame us and we ended up
sub-contracting the job to a local tradesman. The shear weight
and effort involved to cover 11,000 square feet of wall would have
killed us. It was an interesting experience to try to teach myself
how to mix and apply stucco, though, so I've left the information
below pretty much intact in case anyone else might be interested in
doing their own small job ... or stupid enough to tackle a large one.