I was recently involved in a discussion on the forum of paving expert. The topic of the discussion was the sealing of natural stone paving and in particular sawn sandstone paving. While not advocating the sealing of all stone paving, London Stone do recommend that softer varieties of sawn sandstone are sealed. The reason for this is because when paving is sawn the pores of the stone are left exposed. This results in the stone becoming more porous which in turn can lead to water absorption and discolouring over a period of time. The softer varieties of sawn paving such as beige are susceptible to this. As a result London Stone recommend that our beige product is always sealed.
The debate on the paving expert website discussed whether the undersides of the stone should also be sealed, the reason being to prevent cement residue leaching through and staining the surface of the paving. It sounds like a good idea until you begin to consider the implications of sealing the underside of paving. The function of stone sealant is to prevent water from being absorbed into the stone. On the flip side the key to paving bonding with the bedding layer is water from the bedding layer being absorbed into the paving. If the underside of the paving is sealed, adhesion between bedding layer and paving will be compromised.
If you do need to seal the underside of the paving then it is essential that a bond bridge is used to improve adhesion between bedding layer and paving