A coating consisting of an aqueous emulsion of epoxy resin and a polyamidoamine hardener has been developed which effectively strengthens glass containing large flaws. By substituting a silane coupling agent for some of the resin the deleterious effects of ageing can be prevented; the strength of samples aged for two months at 52degreesC and 96% relative humidity was not reduced, maintaining a 197% improvement in strength with respect to uncoated samples. X-ray photoelectron spectroscopy showed that the accelerated ageing treatment led to an increase in the concentration of hydroxyl groups in the resin resulting from the scission of amine crosslinking groups. The migration of sodium from the substrate to the surface was also demonstrated. It is concluded that the coupling agent does not render the coating immune to hydrolytic ageing or protect the substrate from hydrolytic attack. It only provides a hydrolytically-stable linkage between the two.