I have a customer that has sealed there grout about 3 years ago. I tried to clean it with grout master and spinner at 1100 psi. I tested a small area with undiluted striper and the sealer didnt budge. She cant remember for sure what she used to seal it but she thinks it was Penetrating Grout Sealer by Tile Guard from home depot. I'm going to revisit this customer on Thursday so any suggestions on how to clean the grout lines would be much appreciated.