Dont know if you get them over there ;but Rotozip are great for glassfibre.
Looks like a mini router but has a cutting tool instead of a shaping tool.(looks like a spiral drill bit ) Nice because you can set the depth of cut to just penetrate so no risk of nicking anything underneath(unlike a jigsaw).No need to drill pilot hole as self starting(just angle the tool).Can "bite" if held too loosly, so firm hold and a piece of guide material taped down if you want a clean straight line. Great tool ,with blades that cut anything from drywall to ceramic tile. Much less chance of gel coat damage than a reciprocating tool