As with the bonding between bricks, it is vitally important to keep external finishes in good condition to reduce the risk of penetrating dampness into the property. This is particularly important with cob walls where the lime render is applied with a micro porous washed finish to allow the underlying cob material to evaporate out water. You should employ a reputable contractor with many years of experience in dealing with many different construction types including granite, shale, sandstone etc. It is often appropriate to apply a double strength Sovereign Water Repellent to porous stone surfaces.