From what I understand, it is pretty much standard practice to brush diluted PVA on to a bare wall and/or backing coat plaster prior to applying the skim coat. Here's a quote:
"[FONT=Arial, Arial, Helvetica]Plasterers dilute it [PVA] to the recommendation on the container and brush it onto the wall. This creates a barrier and helps the plaster stick to the wall. It also prevents the plaster from drying out too fast and so helps prevent it cracking. You cannot normally plaster over finishing plaster without it cracking. Using a coat of PVA means that you can plaster over finishing plaster without it cracking."[/FONT][FONT=Arial, Arial, Helvetica]
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