QUESTION

I have a flagstone/sandstone 16" wall next to my pool. I is eroding and looks terrible. I want to just cover it up by applying a mosaic tile
over it. This will be just a temporary 2-3 year fix until I have the pool resurfaced and apply new coping and fix the wall permanently.

Any advice would be appreciated.

ANSWER

ANSWER - I assume this is an exterior application of the sandstone in a flagstone irregular shape.  Sandstone is Quartz stone from a geological point of view, but it is a class 1 that is the weakest and most absorption of the Quartz stones.

In an exterior application, particularly if it is being subjected to a lot of heat and moisture and isn't directly attached to  a waterproof membrane, sandstone tiles have a propensity to spall and degrade, so it has a somewhat unstable surface.

You could grind the surface flat and then adhere your mosaic tiles to it with a polymer modified thin-set mortar, but if the stone surface continues to spall the tiles will come loose eventually.  Of course the mosaic tiles need to be properly installed.