I've never heard of pregrouting. I don't think that would work, My advise is to adhere your glass to the stepping stone. Allow it to set and then grout after. I think that's the way to do it,
Cindy

I would think you w/pre-grout onto the paper/tesserae. If your tesserae isn't stuck well enough to grout, then I'm not sure how you'd go about applying it to the stone. Perhaps you could turn your preparations over onto the same not-too-sticky paper, then get another sticker paper, turn it back onto that and pre-grout it. Best idea I can offer.

I've only got a smidgeon of knowledge here, but from what I remember from our demo stone (several years back)~

We mixed the plain cement up and put in enough to cover the glass as a first pour, then added the stuff with the pebbles in it on top of that. We had our glass pieces far enough apart that the first layer oozed down between the design pieces.

I don't remember specifics though, sorry.

Appreciate your input. Will probably fall back to the original plan and thinset to the stone and then grout once set. I was looking for a shortcut since I'm doing the designs at home and the stones are located at work. Eliminating a step at work would have made this process a lot easier but the extra labor for a nicer finish is worth it.

Thanks :-)

If there was a method of pre-grouting, I would imagine it to actually take MORE time. Because you would have to be careful not to dislodge pieces as you smooshed the grout into them.
Good luck on your project. Remember to post pics!!

You state 'existing stepping stones' that says to me they need cleaning not pre-grouting, not sure what that means anyway, resurfacing maybe?
Either way, a driveway degreaser should do the job. Then thinset or whatever you have in mind will give you a good bond on a clean porous surface. The last thing you want is the thing lifting after all the work you put into it for lack of surface preparation.

If the contact paper is not sticking to the glass and your plan is to drop the mosaic onto the thin set in a single step then lay a board over your work and flip it, burnish the contact paper to the glass with a plastic fid or popsicle stick. Cut into sections if you still have a problem.
If that won't work then you may have to replace the contact paper or warm it up. I use contact to protect bevels and it sticks like crazy because of the heat generated by the soldering iron.
Setting time will be slower if you don't pierce the plastic and the moisture held in the old stones will also want to hinder the going off time.

Icheer, I believe pregrouting was mentioned in the tutorials on this website, she has a section called classroom or 5 mosaics in progress...

Here is a link that might be useful: hannacroismosaics

Sorry, the stone we made was a from scratch in a mold, wish we could edit old messages!