Run the tip of a sharp No 11 blade along the point where frame and linen meet to sharpen the demarcation between the two. Another point to remember is that only the sides facing the cockpit opening need to be dead accurate, those out of sight don't need to be a accurate. A lot of the stringers in the tripe for instance are out of sight once everything fits in.
Personally I find masking each off more tedious and irksome than freehand painting each one by one, side by side.
Nice tip Des, gonna go see if I can find those beauties tomorow.
Keith