We have decided to tile over our fireplace hearth. The exising tile has been there for 40 years and we are afraid that pulling out that tile will damage the structure Based on advice of local tile store we going to use thinset with acrylic for mortar. Before applyinng new tiles we are trying to rough up old tile surface using a sander. despite hours of sanding and removing some of the surface the tile remains smooth. How do we know when we have rough up the surface enough to tile over?