I apply strips of wide premium masking tape to the wing area oustide the center joint. (The premium masking tape is important - it has a coated surface which the epoxy doesn't stick to.) I then mix up and slather on the epoxy, making sure that no 'ooze' is allowed to run outside the taped area.

Once the epoxy gels and starts to set I carefully peel off the tape, epoxy drips and all. This leaves a clean wing surface and a perfectly straight epoxy join line.

Easy as eating pancakes.