This study analyzed the effects of polyurethane (PU) resin treatments on surface homogeneity, dimensional stability, and finishing performances of medium- and small-diameter softwoods produced in Taiwan. Two-pack PU resins were prepared by combing short castor oil-modified alkyd resin serving as a polyol with polymeric 4,4'-diphenymethane diisocyanate (PMDI) serving as a hardener, by the molar ratio of NCO/OH+COOH of 1.2. Four types of short oil-modified alkyd resins with different polyhydric alcohols (glycerin and pentaerythritol) and polybasic acids (phthalic anhydride and isophthalic acid) were synthesized. Three kinds of medium- and small-diameter softwoods, including China fir, Taiwanina, and Japanese fir with a diameter of 10-15 cm were obtained from Hui-Sun Forest Station, Taiwan. The wood coating of nitrocellulose (NC) lacquer including sanding sealer and top clear was used. Results show that the surface hardness, homogeneity, moisture excluding efficiency, and antiswelling efficiency of woods were enhanced by PU resin treatments. Among all the PU resins, the isophthalic acid and pentaerythritol-containing PU resin (IPA-P-MDI) achieved the best improved efficiency on dimensional stability of woods. Results of two types of finishing procedure, i.e. NC lacquer sanding sealer plus top clear and top clear only, applied onto the PU-treated woods revealed that the hardness, adhesion, and durability of NC lacquer films on the PU-treated wood were superior to those of untreated one, especially for top clear finishing alone. (C) 2008 Wiley Periodicals, Inc.