Thompson’s Water Seal One Coat is a clear micro silicone coating for bricks, concrete or sandstone, designed to protect from weathering, prevents freeze and thaw damage and reduces efflorescence. The water seal can be applied to damp surfaces and has a drying time of 2 hours.  It is suitable for use on brick, concrete and stone.
Prepare the surface with a stiff bristled brush and give the tin a good shake.  Apply by brush or roller, working in 1 metre sections from the bottom up.  Once that area is fully saturated move on to the next one.  Make sure the whole wall is treated, not just isolated areas.