One style of kitchen tile back splash that is currently popular is a mixture of large tiles with an accent strip of smaller mosaic tile as shown in the photo. Often times the smaller accent tile will also be thinner. A professional tile installer who is used to bedding tile in thinset cement would likely have no problem dealing with this. For those of us who do tile once in great while, it can be a challenge to apply enough thinset to keep the tiles flush, and not so much that it doesn't ooze out between the tiles making a difficult clean up problem. I want to share a "trick" that I came up with that may help you if you find yourself dealing with this situation.
In my case I wanted to add 1/8" of filler behind the accent tile so I could use a smaller amount of thinset put in with a square notch trowel. Scouring my basement for a solution I found some left over 1/8" screen spline. Screen spline is inexpensive and comes in a few different diameters. Purchase the diameter you need, and here is how you can use it to build up a flat area to lay the accent tile on.
Step 1: Step 1. Attach the Spline As Thickness Guides
The spline is used as thickness guides in building up the area where the accent tile will be placed. Place a drop of super glue on the wall at a starting point and press the spline into the glue. Hold it in place for 30 seconds using a flat object. I used a pen with a flat cap. Place the next drop of super glue about 6" from the first, pull the spline taught, and press the spline against this next drop of glue. Continue this process keeping the spline taught as you go. It will stretch fairly easily. Cut the spline just short of your final point, and stretch and attach it to the final glue drop holding it with a pliers. Run a second spline parallel to the first. You now have two rails you can use to set the thickness of a coating of thinset.
Step 2: Step 2: Apply a Coating of Thinset
Using a flat blade tool such as a spackling knife, fill and then skim the area flush with the tops of the spline guides. Allow this coat of thinset to harden prior to attaching tile over it.
Step 3: Step 3: Set Accent Tile
Once the fill layer has hardened, you can now set the accent tile in the same way you set the thicker tile without the need to bed it. Apply thinset with a notched trowel as per manufacturer's recommendations, and set the tile. I ran the cement notches vertically as the narrow accent tiles ran horizontally. I used a grouting float to set the accent tile flush with the larger tiles above and below the accent.
I hope this trick helps someone else dealing with this situation.