Chris Plastow
This is exactly my trade . Painting the exterior surfaces will only cause a long term maintenance nuisance and most likely interior dampness ! Yes, the walls do need to be able to allow water vapour to pass through them or a build up is imminent . Siloxane silicone spray is the answer to this problem . A good one is Enviroseal , google it . Only use a water based silicone liquid if the walls are damp or wet . Use solvent based if the walls are dry . Silicone does not block the pores of the wall , it makes liquid water form a meniscus and thereby prevents penetration . It is colourless but will leave a slight sheen for about six months . If this is done properly. , you won't have to do it again . Let me know if you riquire further help . Chris plastow .
