Harry,
On the shower that you installed stone pebbles on the floor, I remembered you used modified thinset as opposed to unmodified for better adhesion. What size notch trowel did you use? I've got one coming up later this month and don't want to overdo and have thinset oozing up everywhere, but I want to make sure I have good contact.

Hey Kman...I really liked that stone shower floor that PistolPete did. Have you considered using epoxy to set the stones? I was thinking if you used epoxy grout mortar, you could grout with it too afterwards.

Didn't think of that. Then again, I have no experience with epoxy (except when I drilled a hole through my gas tank, but that's another story). The supplier is sending me some lightweight modified mortar for the floor, so I figured I'd just use what they give me. I have no idea what color grout they're using, or if I would be able to incorporate the epoxy with that. That gives me something to think about. I'll ask the supplier about the particulars.

Well, epoxy is not an option, so the question remains: what is the best size trowel?

I've also read in Schluter's Installation Handbook that unmodified thinset actually has a better bond than modified when it's fully cured. Would this still be the case considering I'm installing pebbles that are smooth? Or am I still better off using modified? I have a bag of Ultraflex 2 on hand that I was going to use, then let it dry a couple of days before walking on the floor, unless someone has a better idea.

i used a 1/4x1/4 inch notch and back buttered each stone individually, since they varied in thickness. With a mat mounted pebble, use the largest notch that will not squeeze out to the point where you'd see it in the grout. that could be 1/4 by 1/4 or 1/4 by 3/8. Ulra 2 should do the trick. With the pebbles the grout actually does a huge amount of the holding.

Non modified thinset sets up better in oxygen deprived situations, but pebbles allow lots of air to get to the mortar, so modifed gives you a better bond and compression strength.