Think of the fabric yarn as a three-dimensional object. When you print you are covering only the top half of a cylindrical object (at most). When the fabric is stretched you will be exposing some of the bottom half (which the paint might help with) but more importantly be separating and exposing the inside fibers. The best approach would be to first dye the unprinted fabric the correct color to get complete "coverage". The type of fabric, type of dye, and type of printing ink can all react differently so you will need to test things to get a satisfactory result. For example the outside of the yarn may always take on more dye than the inside without some agitation and stretching while in the dye. The dye may resist the ink from adhering properly. Perhaps someone has a "cookbook" example which has worked for them.