26 ■ POPULAR WOODWORKING
Concrete & Wood Table
13
your form. Before caulking, I apply
a layer of paste wax to the inside
surfaces. I then apply a generous
bead of caulk along all seams. Im-
mediately after applying the caulk,
I come back and run a metal ball
tool over the caulk lines. (This tool
is also referred to as a fondant tool,
and is commonly used to deco-
rate cakes.) The metal ball pushes
excess caulk away from the seam,
leaving a perfect line in the middle.
The paste wax applied before caulk-
ing allows the squeeze out to be
pealed away easily after the caulk
cures, leaving perfect caulk lines.
Concrete Time
Now for the fun part: the concrete.
To get a smoother surface right
out of the form, I used a two-part
process where the form is fi rst
sprayed with a thin layer of GFRC
mix without any glass fi bers, before
applying backer layers of GFRC mix
with the glass fi bers. The sprayed
layer is often referred to as the face
or beauty coat. It’s applied using a
drywall hopper gun powered by an
air compressor. Since the largest
aggregate in the GFRC mix is #30-
grit sand, spraying a thin (^1 / 8 "–^1 / 4 "
layer) coat and then brushing it into
the nooks and crannies of the form,
produces an ultra-smooth surface
with very few pinholes (compared to
a typical concrete mix).
After spraying the face coat, I
quickly (within seconds) use a
chip brush to work the mix into
the seams, and brush the entire
surface lightly. Brushing the surface
pops any air bubbles that might be
trapped against the form, further
reducing the chance of pinholes. I
sprayed a second face coat after the
fi rst one fi rmed up, although the
second coat is optional.
The GFRC backer layer con-
sists of the same GFRC mix as the
beauty coat and includes alkali
resistant (AR) glass fi bers. (AR
fi bers shouldn’t be confused with
run-of-the-mill fi berglass, which
would break down quickly due to
the alkalinity of concrete.) The tim-
ing of the backer coat is important.You need to wait until the face coat
is fi rm to the touch, but still wet, so
the backer coat will stick to it.
Since this table is 1" thick, and
my face coat was^1 / 8 ", I measured
out buckets of mix (by volume) to
do two back layers of slightly less
than^1 / 2 " each. And, since this form
has a large vertical surface (the
leg), I decided to use AR glass mesh
(a mesh sheet of the same AR glass
the fi bers are made from), in ad-
dition to the AR glass fi bers in the
mix itself, to support the structure.
The scrim will be placed between
back-coat layers and provides
extra strength. It also reduces the
chances of the concrete slumping
as the vertical parts of the form are
packed in. I cut a single piece of
AR mesh to fi t the form shape, and
then moved on to mixing the fi rst
backer coat.Packing the Form
With the fi rst backer coat mixed,
it’s time to start packing the form.
The concrete mix should have the
consistency of slightly wet Play-
Doh. It needs enough mass to hold
its shape, but it should still be sticky
enough to adhere to the previous13 The fi rst step in the concrete
application process is spraying a
coat of the concrete. Then, you’ll
pack the form with the glass fi ber
and concrete mixture.