Finishes for hempcrete - The Hempcrete Topic: Designing and Building with Hemp-lime - page 411

To control the suction, you must spray the wall with water before plastering. Knowing
how much water to apply comes with experience, but the following approach is a good
rule of thumb.
Before plastering a panel, give the whole area (several square metres) a thorough wet-
ting with a fine-pressure spray, until water starts to run off the surface. As you apply
the plaster, watch out for any areas drying out again and give them a quick spray as
required.
The amount of spraying a hempcrete wall will need depends on how dry the hempcrete
is, which is affected by the length of time since it was cast and by current environmental
conditions. A fresh, or 'green', wall that is only just ready for finishes may only need
wetting down once, since there is a relatively high moisture content still under the dry
crust. A wall that has been left for several months will be very dry, and likely to need
more wetting as you go along.
Sometimes, despite wetting down the basecoat, a very high level of suction is experi-
enced when applying the topcoat (this happens on basecoats that have a very dry sub-
strate behind them). A technique for controlling suction in this situation is to apply the
topcoat in two passes. The first skim of half the topcoat serves to control the suction,
and then the top half can be more easily applied. This is easier than wrestling with a
high-suction background and trying to apply the finish in one pass.
Plastering on to wood wool board
The open structure of wood wool board provides the ideal key for lime and clay plasters.
Basecoat and topcoat plasters are applied in the same way as to hempcrete; the only differ-
ence is that an alkali-resistant reinforcing mesh is required within the basecoat (see oppos-
ite).
If there are any movements or other stresses in the timber frame, these always have the po-
tential to cause movement cracks at weak points in the plastering. When using a permanent
shuttering board, the joins where two boards meet creates just such a weak spot. The in-
clusion of the mesh mitigates the risk of cracking along these joins.
If using a proprietary, or other specific, lime plaster which a supplier recommends as being
suitable for the particular board you are using, you need to make sure that it is sufficiently