At the risk of being wise after the event...
You probably would have been better off to put wider masking tape on your cutting board and then use a metal rule and scalpel/exacto to cut to desired width. The problem with trying to use the existing edge of masking tape is that the "edge" of the adhesive layer has already been exposed to the air and dried out when it sat on the roll so never sits down as neatly as the stuff in the middle of the tape does which is why you have those "feathers" of paint bleed.
It took me several poorly-masked vac-form shells and a book on painting RC shells which explained the above before I twigged.
When I do black-and-other-colour chequerboards on fantasy wargames figures I use a Rotring-style technical pen with black ink to draw out the lines and then hand-paint the black with slightly thinned black acrylic (having first dotted each black section to avoid any "out-of-synch" disasters). You could try this on the shell by using masking tape merely a guide to get a straight line out of the pen.
Coop
You probably would have been better off to put wider masking tape on your cutting board and then use a metal rule and scalpel/exacto to cut to desired width. The problem with trying to use the existing edge of masking tape is that the "edge" of the adhesive layer has already been exposed to the air and dried out when it sat on the roll so never sits down as neatly as the stuff in the middle of the tape does which is why you have those "feathers" of paint bleed.
It took me several poorly-masked vac-form shells and a book on painting RC shells which explained the above before I twigged.
When I do black-and-other-colour chequerboards on fantasy wargames figures I use a Rotring-style technical pen with black ink to draw out the lines and then hand-paint the black with slightly thinned black acrylic (having first dotted each black section to avoid any "out-of-synch" disasters). You could try this on the shell by using masking tape merely a guide to get a straight line out of the pen.
Coop
