Penetrating repellents, mainly silanes and siloxanes, soak a few millimeters into the pores and line them with water-repelling molecules without forming a film. Rain beads off, absorption drops sharply, and the wall can still dry. They do not bridge cracks or resist water under pressure, and they must go on after repairs, since they block mortar and sealant adhesion.
Repair all the joints first, the water repellent goes on last or the sealant won't stick to the stone.
Water beading on masonry looks reassuring, but repellent does nothing for cracks wider than a hairline; those still need sealant or repointing.
Also heard as
- penetrating sealer
- water repellant
Related terms
Efflorescence
White crystalline salt deposits left on masonry or concrete as water moving through it evaporates.
Repointing
Raking out deteriorated mortar joints in masonry and refilling them with new mortar.
Spalling
Concrete, brick or stone breaking off in flakes or chunks at the surface.
Primer
A thin liquid brushed or wiped onto a joint face or deck so the sealant or membrane bonds reliably.