Epoxy ding stick is probably not the best product to use.
It'd a bit hard to get it to fair in well, and it hardens very hard so it does not sand well.
West systems epoxy resin ( 2 part) is probably the best product to use.
Get the slow set hardener and use alot of tape to keep it off area adjacent to the repair.
Can you describe the "grey lines" a bit further.
You can use regular automotive carnuba wax, or get one of the carnuba based boat
waxes that are made for fiberglass boats.
Be careful of products with alot of abrasve in them, you don't want to remove the finish/paint,
just clean off the gray lines.
Are these at the water line i.e could these lines be caused by the water where you are sailing?