Use 3M mylar 'fine line' tape for precise painted lines. Be sure to remove any waxes or silicone from the gel to ensure no 'bleeding' under the tape, press down the tape 'several times' and with 'dwell time' between the 'pressing' to ensure that the tape 'seals'. "Fine Line" is expensive, but nothing can beat it for painting a precise line. When applying keep a consistent pressure on the 'roll' as varying pressure can slightly change the width dimension of the tape.
example: 3M 218 Fine Line Masking Tape