Before bonding agents came out, you only had pva, agreed, so all you could do is go for 2 or 3 coats of pva to seal it. 1st coat of skim let pull in, that will tell you if its going to go wrong, (ie) bubbles, etc,then put the second coat on, The pva must have 2 thick coats, and left to dry, the 3rd coat of pva is for the skim, apply the skim when tacky. Not for the faint hearted !!