I am putting tile over durock in my new shower installation and the
tile guy rightly recommended thinset. However, when I installed the
durock I had different information and floated the seams with some
mastic I had laying around. So, do I have to remove the mastic before
I use the thinset, guessing the answer is yes, but confirmation is
always nice.

Thanks.

Barker9 writes:

So, do I have to remove the mastic before
I use the thinset, guessing the answer is yes, but confirmation is
always nice.

No. Did you use fiberglass joint tape?