A cement fiberboard substrate is treated on at least one edge of the substrate with an aqueous solution containing at least one phosphorus acid or salt of a phosphorus acid. The treated edge is overcoated with a sealer, primer or topcoat. The resulting prefinished fiber cement siding product has improved coating adhesion, especially near burnished regions and near edges and corners proximate the burnished regions. A phosphorus acid or salt of a phosphorus acid also may be added to the sealer to provide further improvement in coating adhesion.