Lime plaster on magnesium board

The first areas we plastered were in Mike Wye 3/2 lime plaster, which we had some problems with (as already discussed).  The next plaster we tried was site mixed lime plaster.  We mixed this 1:1

Lime putty and

Kiln dried sand.  This sand is slightly courser than the sand used in the Mike Wye plaster and seems to go on the boards slightly better.  We are still having trouble getting over the beads and the plaster is still not drying as evenly as it could.  One theory is there are very slight undulations in the board which mean the thickness of the plaster varies very slightly.  We are also getting some shrinkage cracks as it dries out.