Hi, I have been reading the thread with great interest. There appears to be a great emphasis on using a silicone based repellent. Whilst this will work for a couple of years, given our very abrasive environment the surface treatment soon wears off, and you are back with a porous envelope.
Some years ago this was a great problem within the civils in respect of concrete and spalling especially bridges. Myself and a very good friend were in touch with certain chemists in the USA who had developed a new product that not only prevented the water entering the substrate but also allowed the substrate to breathe. I note that this has been raised by a previous commentator.
This is how it works, the product is not a silicone but a hydrocarbon and relies upon the porosity of the host substrate to draw the hydrocarbon into the substrate. We found a penetration of between 25mm and 35mm to be the measured penetration. This then just sits there a bit like the fabric gortex, whilst rejecting the penetration of water allows the exit of contained water and allows the substrate to breathe.
The UK test bed for this was a house high on Ilkley Moor, Yorkshire, and boy does rain drive up there. The outer skin had become totally porous and ingress of water was prevalent. We applied two coats by spraying (you have to mask up all fenestration and vents), and it worked, nothing has indicated that it is still not working and that is 15 years ago. This has also been applied to York Minster and innumerable bridges up and down the country so it is well proven.
The name of the product is 'Thompson's Water Repellent' distributed now by a very large chemical company and available from any good builders merchants. There are other such products that have come onto the market now, but the key word to look for is 'Hydrocarbon', it is that that does the work.