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

"Tom" wrote in message
news:wkWSa.2453$Ye.294@fed1read02...
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