The effect of weathering on Scots pine (Pinus sylvestris) and Norway spurce (Picea abies) treated with unpigmented commercial primers was studied. Structural changes occurring in radial longitudinal surfaces during weathering over a period of 24 weeks were examined using a reflected light microscope and a scanning electron microscope. Micro-checks were aligned parallel to microfibrils of the cell wall or parallel to the axis of the tracheids, and occurred first in latewood. The first larger checks appeared in thin-walled earlywood tracheids near thick-walled latewood tracheids. After 12 weeks exposure, delamination of the secondary wall occurred. After 24 weeks there were some visible macroiscopic checks on the surfaces and very little separation of individual cells occurred. There were no notable differences in checking and surface erosion between pine and spruce. Blue stain colonization was found first in the cracks between tracheids and it was more common on pine than on spruce. The priming of wood did not prevent but retarded structural changes occurring in the underlying wood. The higher the level of the solid contents in the primers, the more likely the occurrence of structural changes was retarded.