Also, Joe, be sure you have thoroughly shaken/mixed the Roberts stuff. It settles quite a bit. Unless the solids get into suspension, you're not going to see the material settle well into the mortar crevices.
...and I concur with a flatter paint patina....and thorough washing of raw plastic castings...
Lots of good advice in the replies!!
Mike CT.....great-looking mortar on the buildings!!!
KD