Finishes for hempcrete - The Hempcrete Topic: Designing and Building with Hemp-lime - page 407

Wet the wall down with the sprayer until it stops taking water in (see box on page 242 ).
Apply the topcoat using the method described as for the basecoat, and rub it back with
a fine rubber float instead of a coarse one.
Once it is rubbed back, go over it with a damp sponge (the big yellow car-cleaning
type) in the same circular motion you used with the rubber float. This removes loose
aggregate and gives a smoother surface.

Rubbing back lime plaster

There are important reasons for rubbing the plaster back with a float. When the plaster
is laid on the wall, the action of the trowel pushes the aggregate into the plaster, leav-
ing a fine, smooth layer of set lime and water, which is less vapour permeable as it
contains very little aggregate. The rubbing action removes this thin layer, 'opening up'
the surface and exposing the aggregate just below it, leaving a surface with lots of tiny
bumps caused by the particles of sand.

In the basecoat, this has the effect of:

• increasing the total surface area for quicker and more consistent drying of the plaster

• providing a better key for the topcoat

• improving vapour permeability between the coats.

Also, in the topcoat, the rubbing action gives an attractive and consistent texture to the
plaster.

Depending on the aesthetic effect desired, floats made from different materials and/or
different techniques can be used.

When applying a render topcoat, a smooth surface is less desirable, since the increased
surface area given by a rougher finish allows quicker evaporation of any water that soaks
into the render after heavy rain. This can be achieved by floating the surface with a wooden
float or a metal grid float, or scraping the edge of a metal trowel across the surface. These
techniques will also give a more consistent colour to the surface of the render, as the ag-
gregate is more evenly distributed on the surface.