I know that polyurethane will stick to epoxy, but will epoxy stick to polyurethane?

After using our kayaks for three years I’ve started to refinish the hulls on my wife’s and my kayak. The original idea was to sand off the polyurethane, sand out the major scratches and reapply polyurethane. After sanding off all of the gloss I see that the scratches are still there. As I recall from the build polyurethane will not cover scratches, so I believe I need to add a new coat of epoxy, sand that even, then reapply polyurethane. I also sanded to (not through) the fiberglass in some places, so I need epoxy at least those places to restore the waterproof integrity.

My questions are: How much sanding do I need to do? If I haven’t removed all of the original polyurethane will a new coat of epoxy adhere? If the answer is No how can I tell when I have removed all of the poly before I hit the fiberglass?