Our stone cottage has been rendered at some time to give a smooth finish which has then been painted. The render is spalling off in places and I would like to make repairs, keeping the smooth finish. I have tried B&Q and our local builders merchant but they can't suggest anything which would give the kind of finish I want. I have tried a mortar mix, about 3:1 builders sand:cement with a plasticiser and waterproofer but it gives rough texture which does not match the rest of the wall. Can you suggest a suitable product? Is it possible to use internal grade plaster and waterproof it in some way?