Your primer depends on what your final paint will be. Each paint company makes primers for its paints. Decide what paint you are going to use and then get the correct primer for it.
You need to mask off your stripe, clean the area completely with acetone or similar, then sand it lightly to give the paint some tooth. Clean it with acetone or something similar again. Make sure that there is nothing there that can affect the adhesion of your paint. Don't rush this prep.
Prime it and then paint it. You'll get the best result with a roller. Pro's usually spray but if you are careful and apply many light coats then you will probably get a decent result rolling it. Make sure that your paint will not eat your roller.
You can use marine or automobile paint. They are quite similar and the sheer stripe is usually not submerged enough to worry.