I would use a good quality matt to seal the walls first. Using slik may cause it to peal on later coats. I would never use PVA as this can often cause bubbling when you apply paint. See the copy below of an e-mail received from Dulux regarding this.
Thank you for your E-mail I can categorically confirm we DON'T recommend using PVA to seal new or bare plaster. This is because the PVA will seal any residual moisture on the plaster and may cause bubbling when it dries. This is also stated in the BS 6150 and our ICI site work instruction.
Hope this helps.
Omar Amjad, ICI Paints Technical Advice CentreICI Paints, Wexham Road, SLOUGH, Berkshire. SL2 5DSPhone: +44 (0) 870 242 1100Visit our web site at
http://www.duluxtrade.co.uk