masking off paint lines?

You've got it backwards ... Cut out the mask and THEN put it on the model. Don't try and put the tape down and cut it in place, the cutting has to be done beforehand.

ohhh; ok makes sense then. must be kinda tough to guess where the mask will need to be though as far as how to lay it out.

Yep, that's the tough part. You have to know ahead of time what shape you want. For camoflauge it isn't too difficult because pretty much anything close will work!

Frisket is very handy for that. It is available at most art supply stores. It comes in sheets and is translucent and self adhesive. You can lay it down (just don't pull the adhesive backing off), temporarily tape it in place, trace what you need, pull it back up, and cut the pattern out.

A trick learned when building RC stuff was, after the tape is applied, first spray a coat of clear before the second color. That way if there is going to be a bleed under the tape, it will more than likely be the clear which will seal the edge for the color.

Ive heard that; but heard it as you spray the last color you masked over; guess itd have to be clear if you masked over more than 1 color.