Once the glue had set and the "cork stones" were firmly attached I proceeded by filling the joints with mortar. I used an ordinary dry wall spackle for that.
I spread the spackle liberally all over, and then brushed off the excess with a tooth brush.
Next was painting and weathering, in three steps; 1) A gray paint with just a dab of blue mixed in 2) Some black brownish color applied more like when dry brushing 3) A dark brown wash.
I'm not sure if the stones have to much of a sheen, and needs to be dulled, or if the sheen is OK. Or perhaps the sheen is there because the wash is not yet completely dry, although it feels dry to the touch. Tomorrow will tell I guess.