George, something which might help when masking - lay the tape over the frame area to be masked, and then trace the outline of the frame, using a retractable - type pencil, with the 'lead' sharpened to a chisel point, by rubbing it on fine wet n' dry paper, or fine sandpaper. Press down as you trace, which will also helps to burnish the tape along the edges of the intended cut line.
Use a new, pointed blade in your scalpel, and carefully and slowly cut along the penciled line(s), then lift the unwanted tape.
For curved areas, I tend to do this using a magnifying glass, to ensure that the tape is in exactly the required place, and to be certain that the blade has cut right through the tape - Tamiya type tape can sometimes 'drift' on the surface, and although a clean cut may seem to have been achieved, quite often a couple of fibre strands remain which, when the tape is lifted, can also lift the tape intended as the mask.