Remove the screeds from the rotary arms of the clarifier and attach 1" x 4" finished lumber to the arms (set about 1/4" to 3/8 below the position of the clarifier screed).  Use the clarifier to sweep in the surface.  This will give you a sufficient finish.  when you replace the screeds, they should sweep ok.  Trying to put a manual finish on the topping usually results in an irregular surface.

Best, Tincan

tincan, my previous post is unclear, what I was wondering about was the finish of the concrete substrate for the grout topping, not the grout topping itself.  I have done more research and will be specifying a raked finish for the concrete base slab, and will be specifying what you have said for finishing the grout topping.  My apologies for the bad post.

Our company does clarifiers all the time.  We've found through many good and bad experiences the best way to finish concrete prior to receiving grout is to just screed it to its final grade (S-1 finish).  Any other treatment runs the risk of reducing the roughness of the concrete (even though it still looks rough) and bond.
It's funny, contractors who won't get off their duff and do anything extra have the urge to trowel finish clarifier slabs.  They learn their lesson when that grout starts to delaminate and they have to replace it.