A water soluble copolymer of polymerizable esters of unsaturated carboxylic acids having the hydrazide group as a substituent comprises a major proportion of monomer units which are methyl acrylate and/or methyl methacrylate and a minor proportion of co-monomer units which are one or more alkyl esters of acrylic acid or methacrylic acid wherein the alkyl group comprises at least four carbon atoms. The copolymer has film forming properties which make it useful for use in protective coatings. The invention includes a film forming composition comprising the copolymer and a cross-linking agent which cross-linking agent is an aldehyde or an organic thermosetting resin containing methylol or methoxy groups, the copolymer and cross-linking agent being dissolved in an aqueous medium in amounts such that they will form a film when applied to a surface. The composition is applied to a surface and heated at 120.degree. to 180.degree. C. for from 5 mins. to 4 hours to produce a water insoluble coating from a water based composition containing the water soluble copolymer.