Originally Posted by Andy M.
Getting back to the OP's question, which is:
How do I clean grout in an existing tile floor.
I replied with the way that works for me. A quick trip to the local home center and you'll have your choice on cleaners, brushes and sealers.
When your floors are installed and if you opt to seal the grout yourself, ask the contractor what is recommended for the grout he/she used.