We are building and extension and its lightweight ytong's on both leaves. Although they are not really smooth there is no scratching was you get on say Celcons or Thermalites. I am rendering it myself, done lots in the past but not sure whats the best preparation fro this block. I have a few choices as the surface is not rough enough and the bricky didn't rake the joints. Mesh, [no thanks], use a proprietary prep mix or scratch it up. I think the last one...? I was going to XXX lightly with and angle grinder or would there be a better solution. Using 4 :1 first coat and 6:1 second with waterproofer.