As a professional, I use this product where existing grout shows no signs of coming loose or cracking. If the grout is grubby or has grease or previous treatment residues, the pre-treatment is a must to sort that out, then, once dry the application of the grout colourant is easy.

Total transformation
5 stars but you must pre treat - this product is awesome see my pics and more to be uploaded once complete.

Had light grout on kitchen floor which over time became grubby. Chose dark grey grout rescue to give a contrast look. Simply painted on with a 1/2" brush and after about 15 minutes wiped off the excess on the tiles with a damp sponge. Did 2 coats and looks are greatly improved.

We were honestly on the verge of covering over our tiled flooring before we bought this product as a final attempt at tackling the discoloured grout. This product has covered it perfectly after 2 coats and it looks so clean and fresh.