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The storage boxes were designed based on the measurements of the mini-shelf that sat on top of the old bathtub. The box was built out of 2 x 4s and plywood, and backer board was installed on the surfaces in order to tile. Use the leftover sheet tiles and bull noses for the storage unit.
Tile the box inside and outside. Apply the thinset and lay the tile sheets. Use the bull nose tiles at the corners. Let the thinset cure for 24 hours. After it has set, grout the tile.
Note: Small inside areas are difficult to tile due to the trowel and lack of space. Take it slow and be patient.
For an extra safety precaution, be sure to screw the storage into the back wall.
Now it's time to caulk. Use a flexible sealant in the areas such as a 90-degree corner, the back of the tub and on the lip of the tub. The caulk should be the same color as the grout so you can't distinguish between the two.
The final step is to seal the grout. It's important that you seal the grout so excessive moisture doesn't get underneath it.