QUESTION

I want to create a mosaic tile scene on my outside wall around my house. The wall is coated with white stucco. Does the stucco need to be completely removed from the block wall underneath, or can I apply anothre surface to the stucco to make that my new base? I want to be sure that the mosaic tile design doesn't fall off after it's done.
Thank you.

ANSWER

ANSWER - You should never adhere anything to a stucco finish on a wall.  The stucco is typically a cementitous material with a low amount of cement that makes it relatively weak.  You should remove all of the stucco from the underlying substrate.

After the stucco is removed, you can use a patching mortar to adjust the wall surface to make it plumb.  Then you can bond to the mortar substrate.  Since this is an outdoor application, I would first apply a liquid applied waterproof membrane, supplied by the same manufacturer of the thin-set mortar adhesive, to the wall substrate and then adhere the tile to the membrane.  Using the waterproof membrane will help prevent or reduce efflorescence staining and reduce the threat of freeze-thaw damages to the substrate.