Project type:

Mineral paint over old Limewash

In the space of less than 10 years, I Lime-washed this cob barn-conversion twice, and the owner Lime-washed it once more. Usually lime-wash will last a little longer than these attempts, but the barn has barge-boards set very close to the gable-end wall surface, so water drips off the boards straight onto the wall. Lime-wash doesn’t mind being damp, but the mechanical action of the dripping water cuts into the soft finish. Fed-up with redoing the lime-wash, the owner asked me to provide a longer lasting solution. I had success with another flaking-limewash job that benefited from Beeck mineral paint. I was also confident in painting Mineral paint over old Lime-wash, and that the high initial cost of the paint would be worth it.

Why use a Mineral paint?

Most of us are familiar with acrylic based masonry paints. They’re cheap, colourful, and easy to use. But they can lead to problems on a traditional building. It needs a breathable coating to let moisture escape. For example, a building that has been rendered on the outside with lime plaster was traditionally often lime-washed. This is because the lime-wash is compatible with the lime render, and it is also extremely vapour permeable. (see Cornish Lime:”Why use Beeck mineral paint?”)

But lime-wash is a soft coating, and is susceptible to weathering, particularly from dripping water, or driving rain on exposed sites. Also, the original application of lime-wash needs care and patience. Too often it has been slopped-on too thick to try and make it cover, and then left to dry-out in the sun, all of which reduces its longevity.

Mineral paint on the other hand, bonds chemically with the substrate and doesn’t wear off in the same way.

Preparation

First a very thorough clean-down was in order. I applied a coat of fungicide to the walls to start killing the algae growing there, then CAREFUL use of a pressure washer to knock off all flaking or poorly adhering lime-wash without damaging the delicate lime render underneath. I made few repairs to the render, and then another dose of fungicide applied to the clean surface to make sure no algae was left hiding in the cracks.

The particular Beeck system I used comprises 3 parts:

  • FIXATIVE. This looks like water, but is pure crystaline finish without the pigment.
  • QUARTZ FILLER. An aggregate suspended in the finish, it smoothes out the texture of a rough surface, and fills minor cracks. Rather like textured masonry paint.
  • BEECKOSIL. The top coat which has the colour in.

Beeck recommend Fixative diluted with water to bind crumbly or powdery surfaces. The lime render on this building was a bit sandy where the rain had damaged it. It goes without saying, the walls were very dry so that they soaked up the solution like a sponge.

Painting

I next brushed on a coat of Quartz filler thinned with Fixative. It leaves a textured finish that really hides the uneven surface texture where the old limewash had flaked off. Also, the “scraping” action of a brush really forces the filler into the cracks so that most fine cracks do not need any pre-filling and were invisible after the application.

Finally the top coat. Much more the consistency of a “normal” paint, two coats make sure everything is covered.

Another successful mineral paint over old limewash job completed. I’ve just got to wait 10 years to prove it lasts at least as long as I said it would!

Published by: Colin Taylor on: June 6th 2020

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