My exterior coat was probably house paint and was falling right off so I totally stipped. If yours is sticking really well then you may be able to just scuff and prime with something designed to go over existing paint and save a bunch of time.
How much you need to remove also depends on the type of finish that's on there. If you're putting a 2 part epoxy on it then the original has to be two part. I checked with StarCraft before I stripped completely and they told me my boat was painted with a 2 part epoxy. That saved me a TON of work since all I needed to do was scuff it good with 80g paper.
80G paper for What ??? seems a bit Coarse..
I would 80g a FG hull for epoxy ..but for paint..erm..or even for primer erm..
YD.