The invention relates to a surface treatment composition, which contains a solvent, a binder and a substance which prevents the growth and spreading of microorganisms, a process for preparing such a composition and a method for protecting lumber against undesirable reactions caused by mould, blue-stain fungi and decay fungi and similar microorganisms. According to the present invention the substance which prevents the growth and spreading of microorganisms at least essentially is formed by a complexing agent capable of binding transition metals. Preferably the complexing agent is an inorganic phosphate compound, an aminopolycarboxylic acid or a salt thereof, a hydroxy acid or a salt thereof or an organophosphate or a salt thereof or a metal-binding protein (siderophore). By using complexing agents as the main fungicidal components of paints it because possible to protect both the painted surface and the paint surface from moulding and blue-staining by means of a harmless, inexpensive substance.