Spring 2011 Struggling with lime in the frost

January 2011 was taken up with the monumental struggle against the conditions to render a rather nice extension.

A hundred and twenty metres of lime stucco onto concrete block.

Thin coats, lots of frost protection and some luck led to a very pleasing result.

A smooth render should be flat. I hate seeing those undulations that become prominent when the sun strikes the wall at an acute angle.

Corners should be crisp, and ruler straight. The finished surface should be floated as smooth as Bath stone.

Achieving the above is not easy, and the most difficult and time consuming thing is getting the smooth and flat finished surface. The choice of sand and lime is extremely important, and having tried hydraulic lime I am not at all sure that it is the right thing for the final coat. I think that even if the primary coats are hydraulic lime the final coat should be lime putty based as the material is so much more plastic and workable.

Corners are no real problem but there is a short cut: get the first side flat and level – use a straight edge to work to it has to be really flat. When this has set hard do the other side – use the straight edge held flat against the previously finished face, line the edge of the board up with the intended finished surface and plaster up to it, trowelling the plaster well into the corner formed by the board and the scratch coat. Once done the board can be slid away from the corner leaving a perfect edge.

Obviously, we are not using corner beads or dowels with this process. We, of course, do the window and door reveals this way as well.

On this occasion there was no ruling to do, which made life a little easier but the ashlar look is always pleasing when done well, the trick, I think, is to get the joints the correct thickness, the edge of a trowel is not the way.

The render was eventually limewashed to provide the final surface protection and colour.

About byrnesurfaces

conserver and repairer of historic surfaces
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