This work describes an investigation into the aerodynamic effects of the painted surface roughness of manufactured wind turbine blades. Fetches of wavy, orange-peel type roughness typical of new blades were applied to two DU96-W-180 models of chord lengths 0.46-m and 0.80-m. Lift, drag and transition location were measured at chord Reynolds Numbers ranging from 1.5x10 6 to 3.0x10 6 . For all cases, the results showed that with increasing roughness Reynolds Number the lift decreases, the drag increases and transition moves forward. These effects appear to increase gradually with the roughness Reynolds Number, up to a critical roughness Reynolds Number between 20 and 25. After this critical roughness Reynolds Number the effects of the roughness on lift, drag and transition become more distinct. The changes in transition are attributed to rapid growth of Tollmien-Schlichting waves brought on by roughness elements of matching scale. It was also found that the decrease in drag is almost exclusively due to the forward shift of the transition region and not due to increased drag on the roughness elements themselves. These findings show that the surface roughness of even new highly loaded lifting surfaces cannot be assumed negligible.