What this video covers
Grout holds mold because it is porous and sits in the wettest part of the house. The clip gives a three-step method.
It starts with hydrogen peroxide sprayed on and left to dwell rather than immediately scrubbed. The dwell time is what lets it penetrate the grout instead of sitting on top of it.
The takeaway
Grout needs contact time, not force. Scrubbing first removes the staining and leaves the colony in the pores, which is why it reappears along the same lines within weeks.
Read the full written guide: Mold on Tile and Grout.