I would have it blasted. Think of the nightmare if you make the effort and still need to go there. What a waste. If the remaining ablative paint were very thin, I would suggest grinding it off, but doubt that well kept ablative is thin enough. Grinding almost assures some hull damage and requires just that many more barrier coats. You want a nice clean hull to apply a barrier and then the VC17, especially since VC17 doesn't like to stick. Great fast freshwater paint though, if done well.
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In the harsh marine environment, something is always in need of repair. Margaritas fix everything.