I think I used muriatic acid to clean my grout, but I am not sure if that is the right acid, so don’t take that as a suggestion, but rather something you can research. I was just thinking about this earlier this week, because I have some areas of my kitchen grout that need some help. What I remember is the stuff is strong, you don’t want to let it sit too long, air out the area well, wear gloves, and I think it does eat away at the grout, so you can only do it maybe twice in a life time, but when I had white tile and white grout in one house it worked very well. People would notice it when they came in my house they could not believe the white stayed white. You can also do a small test in a small area. It’s so long ago I was going to look into it again.