This was a nice job to work on. We always like limestone because it inevitably ends up with a dramatic difference after being pressure cleaned. We particularly like working with natural limestone, rather than reconstituted limestone as it holds so much more character and charm.
This wasn’t a large job, although it was a bit time consuming as we were tasked with protecting this very brittle natural limestone. These were very old blocks and the pointing was about 40 years old.
To say we drowned the limestone in a chemical pre-treatment would be no exaggeration. The more we can minimise exposure to high pressure on frail or brittle surfaces the better. We applied several treatments over the course of an hour or so and then we lightly sprayed it with water to keep the chemicals wet and working. We then went to lunch to really let the chemicals do the heavy lifting.
It was then a matter of passing over the limestone with various nozzles depending on the stubbornness of the growth and how close to the edge of the blocks we were, where they’re easiest to damage.
You can see from the photo at the top, that there was still some mildly visible growth. Particularly for easily damaged surfaces, this is the level we like to get it to before applying a chemical post-treatment. This is because the treatment will do the last bit of work for us, without causing damage. This is the point where you start to hit rapidly diminishing returns by persisting with just using pressure. We will happily opt to spend a bit more on chemicals and not risk damage.
The pre-treatment that we used was a Sodium Hypochlorite, surfactant and Potassium Hydroxide mix which works extremely well. The Potassium Hydroxide can sometimes cause a very light reddening which you may be able to see in the above photo, but that comes out with the post-treatment. For that, we use a Sodium Hypochlorite solution without the Potassium Hydroxide, which would be a bit too aggressive to leave on without rinsing.
The problem we always come up against when pressure cleaning limestone is that we’re very rarely back at a property after the post-treatment has had time to do it’s work, unless it’s a very difficult job where we’ve organised in advance to return and apply a second treatment. This means that we almost never have true “after” photos showing the final product, which is sadly the case with this one too.