Our home was built in 1962 and we have the grout to match! The 1st floor is an AirBNB and we’ve received an average of 4.6 on our cleanliness score, and dammit I want to bump that score up to a 4.8. I’m probably going to do one more pass to spot clean tough areas but generally, in order to do this job I recommend the following:
Mix 1/4 cup of Oxyclean with 2 cups warm water. Pour over grout and let sit 5-10 minutes. Then scrub the crud out!