The secret to painting perfect stripes is to: (Lets imagine you are painting black stripes onto a white wall.)
1. Tape off the stripe with blue painters tape.
2. Paint over the edge of the tape with the base color (white).
3. Let dry completely.
4. THEN paint the black.
This method allows the white to fill in any areas that might have allowed the black to seep or feather through.
Do It And How 