3M fine line tape can be had in almost any width from 1/32" up to about a half inch. You use this tape for the parting line only. You use masking tape to cover beyond the fine line tape. It's availiable at better auto paint stores. 3M also has a version for pin striping. It's a three piece tape that you lay down, then lift the center piece up to leave a perfectly spaced stripe to paint.

Vince