My daughter is cut from my cloth, so she likes to repair everything. She removed the tile from her shower floor and wants to re-tile. OK so far. I plan to resurface the floor to remove the inevitable chunks. I am carefully using a small angle grinder with a diamond blade. Should take every high spot off efficiently. My question: Should we apply a bonding agent to the cement floor before re-tilling with thinset? OR, should we add the agent to the thinset so it will be mixed? OR, do we even need the agent at all?