I am building a P-40 which requires a frequent but somewhat difficult camouflage pattern - 2 color, hard edged, 'wavy line' camo, very similar to Battle of Britain colors on a Spitfire or Hurricane.
My problem has been that since the edge between colors is hard, it needs to be created with a mask, and more particularly with a mask that is right on the plastic surface to prevent any softening or feathering. If the lines to be painted were straight, then tape would do nicely, but because they are wavy, (that is gracefully curved) I find that it is difficult to cut tape that way, even with a curved edged scissors.
After some thought, I came up with the following technique:
1. Make the masks. Since I had a computer, a printer and a scanner, as well as a diagram of the the intended pattern (on a 3rd party decal instructions) I took the easy way and scanned in the instructions (which had a top, port and starboard diagram of how to paint the plane), and then resized the diagrams and printed them at 1/48th scale on ordinary printer paper. The resizing was done by Mark One Eyeball, and required several trials (and errors) to get about right, but its only paper. If you don't have the equipment to scan, you will have to cut the masks by hand, but do them on ordinary paper, not too stiff.
2. Paint the base color (use the lightest color first).
3. Cut the masks. You want to cut the masks from that portion of the diagram representing the base color.
4. Attach the masks. This was the creative part, because the masks have to snuggle close to the plastic. To accomplish this, I coated only the back side of the masks with rubber cement, being careful not to put any on the model side. When done this way the cement is very easy to remove without stressing the paint underneath.
5. I carefully cleaned up any cement that had squeezed out around the edge of the masks. The cement is itself a mask, and where it squeezed out it would not be clean color division. To clean the cement, just gently rub with your finger and the excess becomes a little ball of rubber.
6. Remove the mask(s). This is done by gently peeling them off. The rubber cement is weak enough that it will not pull any paint of the model.
7. Clean of any cement that may be left using the finger rubbing technique.
This turns out to be a fast, effective way of masking for the particular problems of this camo scheme.