interesting.... I'm thinking of trying some of the masking tapes here :
http://www.everythingairbrush.com/ac..._Products.html
The Finespray Fine Line Green Tape, to be specific.
I like the idea (for both convenience & nice parallel lines) of using a masking line tape underneath the liquid mask.
I worked for me pretty well on my recent shell - only problem was the tacky residue when lifting the mask line.
For more complex work, e.g. edging flames etc, then cutting by hand is clearly the way to go.