External stone work has a real habit of shedding paint. It peels, bubbles and dusts off. This is because masonry paint is designed to be waterproof. Stone on the other hand generally isn’t. The stone that we use for mullions, key stones etc tends to be limestones and sandstones. These are porous materials and so if any water does get into the stone (and it invariably will) then it will want to get out again. If it is blocked by an impervious layer then it will force this covering off. Water is a very powerful element in buildings.
So the masonry paint will then be blown off of the structure. However this then lets more water in and this again is trapped by the paint and this can damage the underpinning stone. So your lovely stone features will not only look tatty it will be structurally damaged as well. All this for just not choosing the right type of paint.
Up until now, we have been recommending the use of silicate paint, however it has only been available in 10 litre pots from our supplier – earthborn. After, what seems like years of nagging on my behalf, they have come out with some smaller sized tins. Hooray!
Earthborn now produce a one litre primer (this has to be applied directly onto the stone, so you will have to remove the existing paintwork) and 750ml pots of the silicate paint itself. This is much cheaper and produces a lot less waste.
With a breathable paint on the stone (and it can also be applied to lime render, cement, brick etc) then you will have a long lasting paint that works with the underpinning substrate rather than against it. Marvellous.