Ardex on Gypcrete Subfloor
 

I'm prepping a 3/4" light-weight cement subfloor (could be gypcrete,
but not sure) in a 2nd story condo for floating an engineered wood
floor. Subfloor is in good shape with only small cracks, but it's not
smooth enough, with deviations in some areas up to 1/2" max. (The
woodfloor specs require maxiumum 1/8" deviation over 10 ft). I'm
considering using Ardex GS-4 self-leveling compound, but the minimum
application thickness is 1/4". Ardex has another product for thinner
applications and feather-finishing called SD-F, but it's a portland
cement, and I'm not sure it's suitable for a gypsum subfloor (assuming
the subfloor's gypcrete). Does anyone have experience with either or
both of these products and suggestions as to which would be more
appropriate? Will the GS-4 stick in areas where it's not 1/4" thick?