In addition to the dehydration and drying, the storage stability can be further improved by adding lower alcohols such as methanol and ethanol, or alkoxysilanes such as n-propyltrimethoxysilane, vinylmethyldimethoxysilane, ??-mercaptopropylmethyldimethoxysilane, ??mercaptopropylmethyldiethoxysilane and ??glycidoxypropyltrimethoxysilane.