Molding vertical tile designs

To level out the back, you'll need to add more putty as a second (or even third) layer. For that large of a tile to just to get the design down, one box may be enough but to get the back level and to ensure even coverage, I'd have two boxes per tile on hand - better to have too much than not enough. Be sure to work quickly and mix up small amounts at a time to get the entire design set - Amazing Putty will set in 3 minutes and it WILL get harder to push onto the design the closer to 3 minutes it gets so do small amounts at a time or you may lose some details. You can always add another piece to one that you've already got on the design but if the piece in your hand is close to or past the 3 minute mark, you won't be able to put it on the design. You might could use it to level out the back (to make up some of the bulk) but that's all it will be good for. A dime or quarter sized amount of both sides should be small enough to mix at one time to ensure it's fully mixed and on the design before the 3 minute mark. Also if the room is a little warm, you may not have the full 3 minute work time so be careful of that too.

Now that's just for the regular Amazing Mold Putty with the 3 minute open time so if you can, I'd suggest getting Mold Putty 15 as it has a 15 minute open time for larger projects. The Mold Putty 15 might be a better bet for you if you can get it. I haven't used the Mold Putty 15 personally but I have used the Amazing Mold Putty lots of times and the only difference is supposed to be that the Amazing Mold Putty comes out yellow with a short open time and the Mold Putty 15 comes out blue with a 15 minute open time. Also the Mold Putty 15 comes in a slightly larger amount so you might just need one box per design but again, I like to have more and not need it than not have enough so I'd plan on three boxes per two designs just to be safe. It will keep for a few months if you keep it cool and dark. I've used some that was about a year old and it worked fine for the most part. I hope this helps.