Tears before bed time

I ordered 10Kg of Crystacast plaster to try in the machine. Open3dp's favourite plaster is Hydroperm but I failed to find that in the UK. This plaster looks the business and indeed feels it but there are some initial problems.

It is not spreading properly and the surface is full of tears. I think that perhaps the particles are too fine or perhaps not dry enough? When I watch the powder being pushed along by the roller instead of seeing a tumbling mass like a breaking wave I see actual "waves" being formed. This is I suspect due to the non Newtonian behaviour of the powder, when compressed it acts like a solid, forms the strange lumps then tears up the surface underneath. I sieved the powder, perhaps that was my mistake?