Several years ago we sold finished bed wood boards as an option. Our standard finish was 3 coats of Helmsman Spar Urethane, sanded between coats. Occasionally we had a flaw as you describe, a narrow line right over a grain that just would not accept the finish. No matter how many coats of urethane was applied, it never would bridge across the line. The way we got rid of it was to block sand the board back down to wood and wash the area with solvent such as thinner or the solvent you would use before painting a car. Then wipe it dry and lightly sand again with 180 or 220 paper. Blow out the dust from all the grain areas. Be sure the air is filtered clean and dry, no oil or water in it. If the air has oil in it you will inject the oil right into the grain of the wood when you are trying to clean the grain. Tack wipe the boards and you are ready to spray it again.
It is frustrating, but whenever a finish doesn't adhere to a surface, there is something on the surface that is not compatible with the finish and it must be removed.
Mar-Ktech
