Ok, this drove me nuts for a good week... If you are using a 3D paint package to paint texture channels of a multi-patch NURBS model, you may find that when you import your textures, your edges display artifacts when rendered. Make sure that Wrap U and Wrap V are off for all of the 2DPlacement nodes in your scene. If you are painting Bump, make sure that Adjust edges is turned on for the Bump2D node.