Granite Tile Over MDF

I plan to use 12X12 granite tile for our kitchen countertops. We have installed a 3/4 MDF top over the counters. I have since heard that Type I mastic is not recommended over masonite, etc. Others have told me that Kilz as a primer or scarifying the MDF will allow the mastic to "hold" the tiles. Also, can the tiles be butted and grouted only in the chamfered edges or should there be a tiny gap?
Thanks for and help.
Tom
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you shouldn't use mdf. if you aren't making a mortar bed, you need a thicker base.
you can use exterior plywood or chipboard. you then need to use 1/2" cement board. you then tile to the cement board.
no. use the thinest spacers for granite. 1/32" i believe. use unsanded grout.
regards, charlie cave creek, az
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Having just did this, I agree with Charlie 100%...
You do not want to use MDF for a counter top...
I used 3/4 exterior grade plywood, a layer of thinset, 1/2 wonderboard, a layer of thinset, and then my granite tiles... Install came out awesome... Even built in my own customer undermount sink... Did a nice wood strip on the front of the granite to match the cabinets...
Good luck, Mark

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How did you finish the inside of the hole where the "undermount" sink is?
M.C. somewhere in Ca.
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Mike Columbo wrote:

cut to fit and bullnose the edges. you should consider solid surface granite. allthough i've set plenty on tops, imho granite tiles should be reserved for floors and walls..
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