I'm painting my house at the moment, which has bare plaster on some walls (previously wallpapered) and some walls have been recently skimmed (week or so ago). Does the preparation for painting differ for recently skimmed walls and years old bare plastered walls?
Well, that wasn't very friendly was it?!
AFAIK there is no different treatment needed for old or new bare plaster. I don't know the Zinsser Gardz but I have used several other Zinsser products with great success. The only treatment I have been told of for bare plaster is to apply a heavily thinned "mist coat" of emulsion diluted approx 10 to 1 with water.
I have also been told by Dulux Technical to avoid PVA as a sealer as emulsion paint will not adhere to it properly. HTH.