As usual, follow the Manufacturers instructions for mixing the materials chosen for the project. In addition to the mixing instructions, Manufacturers will include instructions concerning application and estimated coverage. It is the selection of the proper application tools that lead us to a successful job. By following the Manufacturers instructions, we are able to successfully mix bond coat mortars and grouts. Then what do we do with them?
The bond coat must be applied to produce a minimum coverage for the item to be bonded.
This is the reason you will see various sizes of notched trowels recommended for certain sizes of tile. If the trowel size is not sufficient to provide the coverage, a larger trowel size needs to be selected. If 100% coverage is desired, tiles need to be back-buttered. Back buttering is a good practice especially if the tile has a lug back or is an out of plane shape. Back buttering is not difficult. The process is designed to even out the back of the tile prior to setting the tile and beating it in. For some tiles, this may mean only a thin "skim coat" of the bonding mortar is applied using a straight non-notched finish trowel. Other out of plane tiles may require a thicker coat. The idea, again, is to even out the back of the tile prior to setting.
Tile Doctor Tip: For tile larger than 8″ a 1/4″ x 1/4″ notched trowel and back buttering normally leads to 100% coverage. In my opinion, it is best to take the time to use this procedure and eliminate the problems associated with lack of coverage.t
The industry standard for thinset or bond coat coverage, for floor tile, is: "Average uniform contact area shall be not less than 80% except on exterior or shower installations where contact area shall be 95%. Coverage can be higher if the installation specifications dictate. Is it possible to apply "thin set" mortars too thick? Yes it is. Portland cement "thin set" mortars cure by hydration. While they cure a certain amount of shrinkage will occur. Their bonding strength, if applied too thick, can actually fracture tile. This is the reason you will see "medium set" mortars available. Medium set mortar is manufactured for specific installation requirements. It is especially useful when setting tile on irregular setting beds and tile that varies in thickness.
The procedure for applying the setting bed is simple. Without covering or disturbing the layout lines, a sufficient amount of bonding mortar is applied to the substrate using the flat side of the notched trowel first to "key in" the mortar. I then apply enough mortar to comb out the desired area using the notched side at 45 degree to a uniform depth. Be careful to come as close to the layout lines as possible without obscuring the lines. This can be accomplished with a little practice.
Tile Doctor Tip: Do not comb out an area too large if the area can not be tiled completely within 10-15 minutes. If the mortar skins or the surface dries to any degree, it will not bond the tile.If the mortar does skin over either remove the mortar and re-trowel or simply re-trowel the area.