The old grout can be cut out. The mist efficient way is with an oscillating tool with a grout blade. After the old grout is removed then new grout can be installed. This is a time consuming project.
You need a float brush and grout. You want to make sure you have the right kind of grout. For example, you might want a sanded grout for larger grout spaces. Apply the grout with the float at a 45 degree angle, wiping as much off as you go. After it sets, use a sponge to wipe the haze from the tiles.
You can use two different types of grout for bathroom showers. If your tile requires a thin line of grout then it would be best to use a non-sanding type and for tile that requires a wider line of grout its best to use a sanding type.
Depending on the severity of the grout stains, one economical way of keeping grout clean is with white vinegar diluted with water. If this doesn't do the job, you can purchase commercial tile and grout cleaner in any home improvement store. For the most severe grout stains, you can try chlorine bleach, or oxygenated powder, such as Oxyclean or even hydrogen peroxide.
Chip it out clean and replace with a new mix.
CLR is the best product that I have found for cleaning grout. You are able to purchase CLR at most major retailers.
Clean grout can be realized in one's bathtub area, for example, by using the appropriate cleaning agent. A natural and non-toxic way to clean grout is to use water mixed with baking soda.
I have a lot of mould around my bathroom tiles and don't seem to be able to get it out from the grout in between. What grout cleaner will work best for this?
You should use the grout that will work best for the biggest grout space that you need. So for a grout width like you described, 1/16 to 1/2, you should use sanded grout. Unsanded grout will look terrible in a 1/2" grout joint, but sanded grout will look great in both. Use sanded.
The best acid mixture for cleaning grout off of tiles is muriactic acid and water. Great care must be applied when using this acid as it is extremely dangerous. The acid must be added to the water and not the other way around.
The most practical way if holes are larger then 1/2" is to replace the tile. If the space is 1/2" or less sanded grout can be used.
Yes - but you are always best off removing a large portion of the old grout before re-grouting. After letting the new grout dry for 48 hours, consider using a grout sealer to make it last longer.
The only perfect way to repair a tile with a hole in it is to replace it. The second best way is to simply fill the hole with grout. This isn't a pretty repair, especially on dark colored tile, but it will at least stop water from getting behind the tile and turning a small problem into a large problem.