Just did this, boy what a pain in the keester, except my gelcoat was a disaster, and someone painted the boat 10+ years ago (nicely, but it was wore out)...
Then (you'll note I was doing a bottom job too):
Getting better (coat 2).
Again, this gives a good contrast (coat 3):
This is where it stands now... I have some stuff to scrape off of my mess, and the topsides are wet-sanded, ready for the last coat:
If you CAN... preserve the gelcoat, sand 80 grit/150/300/compound/polish/wax. Painting stinks squared. I would not recommend painting to my worst enemy... but that's just me.
EDITED for updating for a finished pic: