I’m replacing ceiling of the garage (in the basement under the living area) with ‘Perlite Gypsum Plaster’ over expanded metal lath.  The resulting plaster is sometimes gray and hard (good) and sometimes white and chalky, softer, less strong.  What am I doing wrong? 

I hand mix (shovel in wheelbarrow) the plaster to a mashed potato consistency.  After a while I developed enough technique to adhere well to the lath. The second coat (the final) and sometimes the first coat may come out chalky white rather than hard gray.  This happens within the same mix batch.  I don’t think the plaster is old or has been exposed to moisture excessively.  The conditions are dry wintry New York, reasonably warm indoors.  Do I need to keep the plaster wet when curing? Am I applying it too thinly so it dries too fast?  Do I need to wet the first coat of plaster before applying the second to prevent it from sucking the water out of the new coat?