The grey side is the one to plaster finish as this is the easiest side to apply the bonding adhesive and plaster to. It would still be possible to apply the plaster finish to the other side but it wouldn’t necessarily provide the same level of results. So, for the best result for your project, we recommend that only the grey side is plastered.There is much debate in DIY forums on whether this is something that matters when it comes to Knauf plasterboard but we believe that the best results are achieved by using the grey side for the plaster finish.