Best way to clean out water jackets

decades ago I remember a post from Doug Dutra the /6 guru & he filled a bare block with a percentage of muriatic acid (the pool cleaning acid mentioned I am assuming) and water & let it set & said the insides came out looking like freshly machined metal? I dont remember what percentage he used & iirc I think he let it set overnight. if it needs to be squeaky clean for the block fill to adhere I would check into that. As you know on these deals (painting/gaskets etc) surface prep is everything.