First, start with a lip template. I draw lips with a freeware CAD program that prints them out in exact size on my standard inkjet printer - or I trace an existing lip. Transfer the drawn lip to a clear plastic sheet, cut it out and voila. I label the templates and keep a library of them for future use.
I do lips one at a time (some glue sheets of PC together and gang cut them). Use an UltraFine Sharpie pen. Here's the trick. Remove the backing from the top of the PC, leave the backing on the bottom, and trace the lip directly onto the clear PC. Wait a minute after tracing the lip for the Sharpie to dry. You don't want to smudge it. If your lip is a model with the line tie out on the surface of the lip, include that point in your template, mark a dot where the hole for the line tie goes, and drill the hole next thing so you won't smudge/erase the dot later while handling it.
I use Wiss metal shears (yellow handle straight cut, about $12 at Home Depot) to rough cut the lips. Cut about 1/16" or so outside the Sharpie line. Now you're ready to sand the lip to final shape. I use a fine grit Dremel sanding drum for this. This is where the aforementioned "trick" comes in. Sanding with a high speed tool will remove polycarbonate but it will not remove its backing, which tends to curl back from the edge you are sanding, obscuring the lip line. That's why you removed the backing from the top of the PC. On the other hand, if you also removed the backing from the bottom, the Sharpie line of the lip would be hard to see on a transparent surface. Leaving the backing on the bottom of the lip gives you a nice solid white background against which the line is easy to see.
After shaping with the Dremel, remove remaining Sharpie ink with denatured alcohol or naptha. Other solvents (lacquer thinner, acetone) will cloud/melt PC, so don't use them. Finally, use a Dremel wool polishing cylinder to polish the edge of the PC. You're done!
You can make very accurate lips with this method. It's not hard, it doesn't take forever, and it doesn't require the dexterity of a brain surgeon. Shaping circuit board is even easier because there's no backing to worry about and the material sands very easily. BTW, I use 1/16" PC and 1/32" circuit board for bass baits.
Edited by BobP, 16 August 2009 - 12:29 AM.