I've been advised the same. I'm due to recoat later in the year and this raises a big concern, as the hull is coated in two pack pitch epoxy. I'd imagine many are in the same position.
My (limited) understanding is that overcoating coal tar pitch based epoxies presents a risk of the pitch 'bleeding' due to the the new coating. If the original coats are compromised by whatever I throw on later, it renders the blacking exercise a bit pointless!
However, a search online found a book (Marine Painting Manual by A.M. Berendsen) stating that this risk when overcoating may be addressed by applying a 'sealer' or 'tiecoat'. I've yet to find a suitable product to do this though.