Finishes for hempcrete - The Hempcrete Topic: Designing and Building with Hemp-lime - page 405

Leave a small amount of plaster in your bucket covered with a damp cloth, which you
will use when you come to float the surface.
You are waiting for the surface to set just hard enough to rub back with a foam rubber
float to give a grainy texture. You don't want the plaster to set so hard that this be-
comes difficult to do. The time taken for plaster to firm up depends on the ambient
temperature, and how dry the hempcrete was when you applied the plaster. At 8 ℃ on
quite a 'green' (not fully dry) wall this could take 24 hours; at 16 ℃ on a dry wall it
could be 30 minutes.
To test its readiness for floating, return to the wall intermittently and press the plaster
with a finger: if this doesn't leave a print, the wall might be ready. Try rubbing your
float (a coarse rubber float for basecoat) tentatively on the surface in circular motions.
If it begins to give the surface a texture like coarse sandpaper, without moving the
plaster on its background or leaving deep lines, then it is ready.
You will find that floating the surface is all in the timing. If it takes a lot of effort to
create the desired surface, then you have left it too long. If this happens, you can wet
the wall down with the sprayer and try again, or use a hard plastic or wooden float.
Once you have 'got to' one wall at the ideal time, then you should be able to gauge the
drying time for the rest of the walls, as it should be similar.
Continue rubbing the surface in a circular motion, keep your float vertical and parallel
to the wall at all times (i.e. don't dig the corners in).
You are trying to achieve an even and rough texture across the whole wall.
Where there are any high points or lines left by your laying-on trowel, you should be
able to take these off using the float.
Any low points (dips) will become apparent, as the rubber float will not come into con-
tact with them and so won't roughen the surface. You can easily fill these dips by
adding some of the plaster you kept in the bucket to your rubber float and applying it to
the dip, continuing your circular motion.
Leave the wall for around six days before attempting the topcoat.
When applying a render basecoat, the rough surface alone is not sufficient to provide a key
for the topcoat, which is thicker and heavier than a plaster topcoat. Therefore, after rubbing
back with the float, the surface of the basecoat is scratched with either a three-pronged
scratcher made from pieces of lath, a comb scratcher or a devil float, to provide a better
key.