To improve the weatherability of a wood by a method wherein the swelling resistance of the wood in its contact with water is improved.

This composition includes at least one (meth)acrylate monomers represented by the formula, in which R1 is hydrogen or a methyl, R2 is at least 12C alkyl or alkenyl group, being 55 to 99.9 wt.% of a total polymer, and at least one polymer, which contains one or more polymer copolymerizable with the composition shown by the above formula as a polymeric unit, being 0.1 to 45 wt.% of the total polymer so as to lower the swelling of a wood in the presence of water. Further, the rate of exudation of a biocide out of the wood can be reduced.