Painting pinstripes on a gas tank might seem difficult, but with the right tools it is very easy. Using just tape, your gas tank's present paint job and a new colour, you could have your new stripes by tonight.
Cut painter's tape into narrow strips, or buy it pre-cut. Aim for 1/8 to 1/4 inch wide strips, for best results.
Begin laying the tape strips over your gas tank, making sure they form evenly spaced, parallel lines. If your tape is thinner you can leave less space between stripes, but leave more if it is thicker. Try for a ratio of 3 to 1 (¾ inch in between every ¼ inch of stripe). Be particularly careful when curving around the tank.
Paint over the entire gas tank with the colour of your choice.
Allow the tank to dry.
Peel off the painter's tape.
Cover the tank with a finishing coat of a sealing product such as carnauba wax, sealant or cleaner wax.
Allow to dry and enjoy your new stripes.
Have fun with your stripes. They don't need to be straight; you can try making them curve in wavy patterns.