A process for improving the adhesion to a solid surface of a heat-hardenable organopolysiloxane having only hydrocarbon groups as substituents and containing less than 0.2 mol. per cent. copolymerized mono-organosiloxane comprises coating the surface with an organo-silicate or organo-siloxane of general formula: <FORM:0711374/III/1> where R and R1 are alkyl groups or halogen substituted alkyl groups, and R1 may also be an aryl, aralkyl, alkaryl, alkoxy or aryloxy group or halogenated derivative of one of these groups; the organosilicate or organosilane being hydrolysed before the organopolysiloxane is applied. In the above formula R and R1 may be methyl, ethyl, propyl, isopropyl, butyl, amyl, isoamyl, octyl, isooctyl decyl, dodecyl, or beta-chloroethyl radicals. R1 may also be of formula -OZ, where Z is an alkyl group as described for R, or additionally Z may be a phenyl, tolyl, xylyl, benzyl, phenylethyl, naphthyl, anthracyl, or diphenyl radical, or halogenated derivative of these radicals. The initial coating may comprise tetraethylortho-silicate, t-butoxytriethoxysilane, methyltripropoxysilane, beta-chloroethylorthosilicate, t-butoxytriethoxysilane, t-amyloxytriethoxysilane, dit-amyloxydiethoxysilane, di-n-butoxydiethoxysilane, n-butoxytriethoxysilane, 2-methoxyethylortho-silicate, di-t-butoxydiethoxysilane, methyltributoxysilane, methyltripropoxysilane, methyltriisopropoxy-silane, di-t-butoxydiphenoxysilane. The organopolysiloxane may be one described in Specifications 594,481, [Group IV], 618,453 and 640,067, [both in Group IV (a)], and may contain, as fillers, silica aerogel, titanium dioxide, lithopone, zinc oxide or iron oxide, as well as cure accelerators, such as benzoyl peroxide or tertiary butyl perbenzoate. The initial coating may be hydrolysed by treatment with acid fumes, or by acid-forming metallic salts, such as Fe C13, CuC12, AlC13, SnC14, either on the surface prior to the application of, or incorporated in, the initial coating; or the first coating may have incorporated therein chlorosilanes or polyorganohalogenopolysiloxanes, which effect hydrolysis with moisture in the air. After the first coating is hydrolysed, washed and dried, the organopolysiloxane may be applied with heat and pressure to cure it, which curing may be performed in several stages. Specifications 597,178, [Group IV], and 676,464 also are referred to.