Some good advice here, some not so. At the start of our project
we were recommended to needle gun the whole boat, or acid wash it, or apply Ospho. In hindsight, I would've disregarded most of that info and just gone down to the local fishing dock
for a discussion with those guys. They rely on their boats 24x7 and know what to use.
We ended up having our steel motorsailer
blasted back to bare metal over the course of 10 weeks (don't ask) and for those areas they couldn't get to with the grit, I hit the existing paint with a wire wheel
to take all the loose / flaky paint off, faired it with 2 part epoxy filler, then painted it all with a 2 pack Jotun paint system.
J87 Aluminium for below the waterline - three coats
J87 standard for above the waterline - three coats
Hauled out after three years in the water
(two years of that in a mud berth) and everything is still hunky dory.
Bear in mind that's for a usable boat that goes cruising. If you're looking for a glass marina-queen finish, be prepared for some serious ££. Here's a pic of how ours turned out: