PURPOSE: To provide a coating compsn. which can be cured at a low temperature and can improve the surface profile of plastic moldings, by adding dihydroxydimethylsiloxane to an organotrialkoxysilane to hydrolyze the latter, and adding guanidine.

CONSTITUTION: 0.02W2pts.wt. α,ω-dihydroxydimethylsiloxane is added to 100pts.wt. component (A) consisting of an organotrialkoxysilane alone or a mixture with a tetraalkoxysilane to hydrolyze component A. Then 0.1W5pts.wt. compd. selected from guanidine, an N-alkyl-substd. guanidine, a carbonate thereof and a calboxylate thereof is added thereto. The tetraalkoxysilane has an effect of improving the hardness of a coating film. α,ω-Dihydroxydimethylsiloxane has an effect of improving mar resistance.