High-density and high-resin-content boards were produced by phenolic resin impregnation into board materials prepared by the water-vapor-explosion process (WVE) to develop high-durability wood composite boards for exterior use. Wet-dry cyclic tests and accelerated weathering tests were conducted, and the fundamental properties were determined to examine the effect of resin impregnation on board qualities. Bending and internal bond strength of resin-impregnated boards (I-board) satisfied the criterion for 18-type particleboard described in JIS A 5908. Thickness swelling (TS) after 24-h water immersion was approximately 2%. Resin impregnation improved the dimensional stability of the boards. In wet—dry cyclic testing, TS of I-board was the same as that of plywood. The retention ratio of modulus of rupture of I-board was large; thus, I-board had high bond durability. Color change of I-board was less than that of ordinary particleboard after a 500-h accelerated weathering test. I-Board had lower surface roughness than boards produced by a spray application method (S-board) and higher water repellency, although the difference in resin contents of the face layer was small. Thus, it is suggested that the surface properties and weatherability of I-board were improved by impregnation of phenolic resin. High-density and resin-impregnated boards made from the WVE elements are expected to withstand actual exterior use.