Need advice on custom paint jobs.

For curved and detail work I use 3M fine line tape it is available from 1/16 to 3/4 inch. Place that down first then apply the masking tape and mask half way over the the fine line tape. (The fine line tape must be the contact point where you want the break in color.) If you thin the paint where you are going to spray near the color break it will not leave any edge for the color change. Thus the name Fine Line Tape. Do not allow the paint dry all the way to un-tape remove the masking tape first leaving the fine line tape. Then go back and remove the fine line tape pulling it up and toward the area with the fresh paint.

If you let the paint dry it runs the risk of lifting the paint you just put down.

Paint too thick will and it will lift with paint strings still attached to the tape. 

Thank you for these replies. Wth the use of fine line tape, I believe the paint break can be "cleaned up" without having to start over.

"Thank you for these replies. With the use of fine line tape, I believe the paint break can be "cleaned up" without having to start over."

Most likely - and I do not think that your job is "bad" - it needs finessing. It appears to have a prototypical shape (from my memory) and appears to be nearly symmetrical - which is part of the finessing.

Here's a Weaver FA-2 that I painted several years ago using plain masking tape and a method similar to yours (no CAD anything - just photos, a template, a pencil, an X-acto and eyeballs). The "blue tape" is not as good as the regular "tan" masking tape, and the fine line tape is better. I'm proud of and happy with mine, though I see flaws - that yellow/red interface is a bit scruffy - nothing that proper weathering wouldn't cure. 

Dave has great advice. He talked me through my early days of custom painting. Good tape and a whole lot of burnishing will give you great paint lines.