Idealy youd use proper gelkcoat which you paint on but the plastic padding gelcoat filler does just as good a job in less time.(make sure you use the white hardner to prevent yellow repairs)once it has properly gone hard then work off the sharp edges with a file (be careful tho) of 250 grit.Then as the repair baecomes more and more flush work up the grades, ideally finishing with 12000 by lightly sanding the whole blade to get it even. Then to get a good repair polish with teflon boat polish to get a shine and smoothness.
Good luck
Ben