The method I use on f/g is to mask up the area to cut and mark your lines on the masking tape. Cut the f/g with a cengar saw
http://www.cengar.co.uk/pages/e-range-JP901.html leaving about 3mm from the finished line and dress the cut back with one of those sanding rolls
http://greens-garage.com/images/drumsander1.jpg working along the length of the cut being carefull not to flick the gell coat off by turning the drum roll against the line. Works for me. have a couple of practice stabs at it in the middle of the panel thats coming out.
Bob