The HG grout cleaner is a ready-to-use solution for removing persistent dirt from grout. It's effective at tackling tough stains, but any remaining yellow or brown marks are likely due to lime scale, which can be addressed with HG's professional lime scale remover. For black deposits, consider using HG mould spray. To use, dilute the cleaner with lukewarm water (1:4 ratio), apply with a hard broom or scrubbing brush, and let it work before rinsing with water. One 500ml bottle is suitable for 10-15 square metres of wall or floor surface. It's safe for all types of grouting and comes in a 500ml size.