Adhesion of a thermosetting powder coating to a glass substrate surface using a silane adhesion promoter, is enhanced by modifying the pH of the glass surface to between 3.5 and 5, prior to, or during, the action of the promoter. The pH-modification may be effected during washing of the surface prior to spray-application of the promoter, or by including an acid or alkali modifier in the silane solution. Alternatively, the silane, or the silane and the modifier, may be included in the thermosetting powder to become active during fusing of the powder to the glass surface. An acid or alkali solution dried on the surface prior to application of the promoter for activation with the promoter on heating, may be used instead or in addition.