We are doing up a 1870s house in Deal, Kent and we have issues with damp getting through the external solid walls. The walls have historically been rendered and painted (last painted about three years ago) but are obviously letting water through. There are no obvious cracks in the render. We would therefore like to paint it with something that will make it waterproof (and seal any small cracks that there may be in the render). It is a three storey house so quite a large area to cover. What products would people recommend for this job - that will result in waterproofing the solid wall without costing us an absolute fortune. Thanks for any advice given.