Probably 20+ year old original multiple layers of ablative bottom paint are no longer adhering to the FRG/barrier coat. If you dont let ablatives fully 'ablate away' before applying continual 'new' coats on top, this will occur after 6-7 yrs. in my experience.
On my boats to prevent/remedy such with 'old' ablatives is:
1. sand in those areas where the paint lost adhesion (in the 'holes') and 'fill' with thickened ablative paint, using a polyethylene trowel and building up successive layers until the 'valleys' are completely filled and fair to the surrounding paint - this 'might' prevent release for a year or two as 'filling and fairing' sometimes 'works', sometimes it doesnt.
2. Best (as Ross states) is to 'bite the bullet' and sand it down to FRG/barrier and 'start-over' with new bottom paint; not 'just' the area where the paint released but 'whole and entire large sections'. Ive found over many years that thick accumulated ablative paint will only 'adhere' to especially barrier coat for 7-8 years and then you begin to get 'blow outs of lost adhesion'. Pressure washing with 'extreme pressure' will discover which sections need to be drastically remedied and which can take 'filling and fairing' (only to be drastically remedied a few years later).
BTW -- after sanding down 'whole / large sections' put the first paint coat on in a contrasting color to your final 'choice' color. Only repaint when the first or 'signal' color is starting to show through; if the signal color isnt showing through, then suggest that you sand the 'choice' color layer .... but dont PAINT it. That will keep the 'thickness' down to acceptable dimensions, etc. All the previous applies to the ablatives that can withstand long term haulout without loss of their anti-fouling effectiveness such as Micron CSC, etc.
hope this helps ;-)
