Make sure the ruler or other straight edge (and any French Curves, etc., ) are "raised above the surface". If the edge of your drawing device is flush with the surface, the ink can, and will, smear under it. It seeps under the edge. It can even do it on masking tape.
The "edge" doesn't have to be real high (most rulers have a cork backing for this) but it cannot be in contact with the surface.
If you are having trouble with the ink adhereing to the painted surface, wet sand it with 2000. A perfectly slick surface is not wanted, the ink will usually bead up and not "stick".