A plurality of workpiece-holding spindles is arranged, each engaging an article being processed, and a plurality of trays, each removably engaging a plurality of said spindles. The trays are moved along a feeding line extending along a feeding path passing near a decoration module. By a transfer assembly, the spindles carrying the articles are transferred from the trays on the feeding path, to the decoration module. A decorative treatment is applied to the articles carried by the spindles, in correspondence with at least one treatment unit (40, 41, 42) operating along the decoration module. In correspondence with a painting module operatively connected with the feeding line, the articles carried by the spindles are painted.