Thinset is fine, also I have used marine silicone adhesive caulking, but thinset isn't as smelly!ha! Are you gonna use dishes and/or stained glass?

I have mosaiced metal trays and watering cans and I didn't use a primer. They were for indoor use and I used Weldbond but thinset would be better. MAC glue or GE Silicone II for windows and doors are also possibilities.

i am going to do glass on it. one book said to use expoxy. but that is stinky. i may try the ge II. I have never used it before. how do you apply it? or i may use my trusty thinset. it is for indoor use. i will take before and after photos. thanks