More than 25 years ago aqueous mixtures containing methylethoxy siloxanes were used which cross-linked within the stone making it water-repellent. Since the storage stability of the made-up solutions was strongly limited, the system was unsuitable for widespread use. Awareness of the environment demands the replacement of organic solvents. It is now possible to offer aqueous water-repellents based on established silicon-organic chemicals. The ready-to-use solution has to be continuously prepared and immediately applied to ensure constant quality. The system must be physiologically harmless, and this is possible when ethoxy-functional alkyl silanes are used. Also they form the proven methyl silicone water-repellents. A continuous mixing/application apparatus is proposed, so that the operator is not faced with problems of varying pot life and the requirements of different substrates, at the same time ensuring that the material is used up before the onset of turbidity.