Ok, I am on day number 6 (if you look at it in 8 hour day working chunks) of scraping the bottom paint (all 30 years of it) off my boat (pictures below)...
My keenly UNPROFESSIONAL opinion is, that bottom paint sticks fine to a properly prepared bottom. Meaning, roughed up Gelcoat, primed using a decent barrier coat, will allow adhesion just fine. Requiring stupid lots of man hours to get it off even after 30 years.
The places where paint came off easiest, is where the prior owners, just slapped paint on top of exposed gelcoat, where the barrier coat failed, or where there was damage, or (love this) where the pads were that the boat rested on (see square marks on the hull).
Where you see pink, it's the first layer of bottom paint on top of white barrier coat (apparently was red).
Where you see blue (ablative) is where someone patched portions of the boat, sealed with gray barrier coat, and painted ablative on top.
Where you see gray, it's repairs (fairing) added to the keel.
From this you can see where bottom paint has failed over the years.
For the record, I'll be sanding down to gelcoat, fairing, barrier coat in gray, then (since I am freshwater) doing VC-17 in 2 colors, Red and Original Copper. Yes Copper is an additive for our paint as well (as powder).