28 ■ POPULAR WOODWORKING
Concrete & Wood Table
Practice Patience
17 Don’t remove the form immediately post-curing. I fi nd it easier
to do the rough grinding while the top is still supported by the form.
18 If there are air pockets or gaps in the concrete, you can apply a
slurry mixture of the concrete to fi ll things in.
19 Wet sanding the top gives a silky-smooth surface.17
During my build, the back coat didn’t go quite
as planned. When hand-packing GFRC up a
vertical surface, you want to pack in layers
of^1 / 2 " or less, allowing enough time for the
previous backer layer to firm up before packing
the next one. I was squeezed for time (with
dinner plans), and the concrete started to
slump as I added the second layer. I was
able to save the project, but I’ll also be telling
you what you should do to avoid some of the
moments of panic I experienced.
I mixed the first backer coat, spreading it
up the vertical surface by hand. It was going
well. I then added the sheet of AR scrim I’d cut
earlier and massaged it into the back coat.
At this point, the vertical was packing nicely
and not slumping, and I still had 3^1 / 2 hours
before dinner. So I waited...and waited...
and 2 hours later I realized my error. The
temperature had dropped outside and my
shop was now 65°. The cold greatly increases
the cure time of concrete.
An hour and half before dinner, when the
first backer layer still wasn’t firm enough,
I realized I had to make a choice between
rushing the project and upsetting my sig-
nificant other. We all know this isn’t really a
choice. So, I decided to go for it and mixed
the second backer layer.
As soon as I started packing the second
backer layer, I knew I was in trouble. Gener-
ally, if you time your back layers correctly,
GFRC is easy to hand pack up verticals
without a back form. However, since the
first layer hadn’t firmed up enough, the
weight of the additional layer caused the
vertical layer to slump. I knew I had to do
something, so I quickly created a make-
shift back form by grabbing a scrap piece
of melamine and clamping it against the
packed concrete on the waterfall leg. This
resulted in a couple small issues: tear away
and glass fibers visible through the surface
in a couple places, and some extra time
grinding the back of the waterfall leg after
demolding. Fortunately, these issues were
fixable with a bit of slurry later on. The moral
of the story is to remember that concrete
cures much slower in cold weather, and
plan accordingly.18
19