In looking at Paul's site--the section on masking off buildings--plastic or otherwise during painting--I was reminded to tell you all about a great new product on the market--a painter's tape called Frog Tape.

Unlike traditional blue painter's tape, you really CAN GET clean sharp lines with it; here's the site with videos. I used to paint a chalkboard on a wall for a friend's office and the lines were SUPER SHARP

http://frogtape.com/index.php/how-to-gallery

Guess I'll have to try it again. I have some I was using on room painting and it didn't blow me away. I'll try it on my next model masking job.