I wish to skim over a wall which currently has several coats of emulsion on it. Plasterer suggested that I coat the surface with PVA (he suggested undiluted !). He will then apply another coat of PVA immediately prior to applying the skim. Is this sufficient preparation to prevent the skim blowing soon afterwards ? I suggested scoring the surface or mixing sand with the PVA but he said neither was necessary. I have had other walls skimmed over existing plaster and have often had lots of hairline cracks appear once the new surface has been painted so I'm worried that his suggestion may not work. Any views ?