http://www.bobvila.com/articles/grout-repair/
Most grout repairs are at a kitchen sink area, floors, and shower floors.
This first step in considering a grout repair is to inspect the area closely. You want to tap on surrounding tiles and listen for any rattling sounds or visible movement.
Step 1 Inspect for movement:
If you see or hear movement, the tiles are most likely not bonded and need to be removed and bonded again. Upon the removal of the tile, inspect the surface for any noticeable damage or signs of deteriorating. Push down on the area and look for movement. Rule: Tile needs to be bonded to a solid surface, anything with movement will cause the tile to break free again in a short amount of time. A common reason why grout cracks is because of movement. Grout is not flexible so any movement will wither crack grout or tile. Corners are very common areas where grout cracks and that is why most people use a caulk because it is flexible. It’s also garbage material and very cheap so plan on replacing that stuff especially in wet areas like shower floors.
If there is movement, then you have a more serious issue and should hire a professional to repair. If it looks solid, then cleaning and scraping the area clean so the new tile will sit flush with surrounding tiles is the next step.
Tools:
utility knife, grout saw, scraping tool like a stiff putty knife or equivalent, vacuum, grout sponge, micro fiber towels, 2.5 gal and 5 gal buckets for thinset and water.
safety equipment: glasses and dust mask
Step 2 – Prep area for new grout
After determining if the area can be successfully repaired with new grout, it’s time to clean and prep.
you want to remove any loose grout and at least the top layer of grout. Grout is designed to get it’s strength from the full depth of the grout joint and wrapping around the tile to form a mechanical bond. Removing the top layer is bare minimum, so the deeper the better. Grout is not an adhesive so applying directly over anything will fail almost immediately. One exception is epoxy grout.
vacuum any debris and wipe area with a damp sponge or micro fiber towel. Nothing sticks to dust or dirt. The goal is to try and make the area look like it’s a new tile install. clean and ready to go for grout. This gives the professional result. cutting corners and being lazy will result in a rookie job.
Step 3 – Choosing the right grout
there are 2 types of grout mostly. Unsanded and sanded cement based grout. That just means there are no fancy additives that claim extra strength, built in sealers, water proofing or mold preventive.
unsanded grout is used for joints up to 1/8″ wide and soft stone like marble or glass which has a high polish and can be scratched with sanded grout during installation.
Sanded grout is used for joint 1/8″ and larger. Tile, granite, porcelain are examples.
no need to use fancy grouts but yes they will work. definitely don’t use any pre-mixed grout. That stuff is garbage and low quality material that is designed for the lazy repair person.
Then there is epoxy grout. This stuff is the best of the best. however, don’t plan on expecting any type of color matching because epoxy grout and all other grout is made up of different ingredients. Use could mix a small amount of Linen grout by custom building products with water, let dry, then mix the linen with epoxy and you will have a completely different color. Epoxy gives a sheen almost plastic appearance unlike the raw texture of regular grout. So you have to decide if you want a bullet proof repair that doesn’t match or a decent repair that will match more. That is for you to decide. I do both depending on what my customer’s concerns are.
Mixing Grout
This is the easy part. Once you determine the type of grout needed for the repair such as non-sanded, sanded, or epoxy, the next step is to mix it correctly. Always follow recommended instructions directly from the manufacturer. Non sanded and sanded grout require clean cool water. Do not use warm or dirty water. There is a step to let the grout sit for at least a few minutes after the initial mix. This is to allow all the materials of the grout to absorb the water completely. Some call it “Slake time”. After the slake period, you will mix again but do not add water as this will make the grout much less stronger and weaker. You will need a clean 2.5 gallon bucket to mix the grout, 5 gallon bucket for your wash water, and a clean grout or tile sponge. They are generally the yellow ones you see in the tile section of Home Depot, Lowes, or Floor and Decor. You need a margin trowel to mix the grout, a rubber grout float to apply the grout and a clean microfiber towel to remove any grout haze after the grout dries.
Applying the Grout
Using a rubber float is preferred when grouting larger areas like a complete shower floor. You can get away with you fingers if you have to if it’s just a few minor trim tiles. Use nitrile gloves when grouting to keep your hands from drying out due to the cements in the grout that suck moisture and oils out of your skin. If you don’t have gloves, wash your hands with vinegar when done for the day as that will eat away the cement residue and restore your hands back to normal. Rinse thoroughly afterwards with clean water and soap.
The goal is to apply the grout and fill all voids completely. You don’t want any air pockets and that leaves a weak joint. Go over the joints in different directions to fill those joints completely. Only apply enough grout that can be washed in anywhere from 5-10 minutes after applying. Time is really dependent on how fast it starts to dry. Warm weather will accelerate and cold will slow down this drying process. You want the grout to start to firm up a little before wiping with sponge. If you get on it too early, you may wash out those grout joints too much. Waiting too long will make you have to scrub more to get the residue off the tile. You also need to know that stone such as marble, travertine, granite and slate are more sensitive to grout. Slate is the hardest to grout imo because it is so rough in texture and very absorbent material. You have to apply a grout release to any type of material that sucks the moisture out of grout in a fast manner. The grout release can be a high end type of grout sealer like an impregnating version such as Miracle 511 sealer. Follow their steps and you will be rewarded if you do with a professional result. Not using a grout release on very porous materials will cause the grout that you just applied to “burn” into the surface which can be permanent. This will result in a very hazy appearance that can’t be undone in some cases. Acids can’t even be used on some types of materials like calcium based stones such as travertine and marble. Acid will permanently etch those surfaces. You can use muriatic acid on slate though and some installer will make a very light solution mixed with water like 10 parts water to 1 part acid. Always pour the acid first into the container then add the water, doing it the other way will call the acid to spit out and if it gets in your eyes, it can cause you to go blind to know this! use eye and hand protection whenever using any acid. use a respirator too, do not breath that stuff, it can kill. I have used muriatic acid for years and it is nasty stuff, but just be careful and understand the dangers.
Epoxy grout is part adhesive and part grout, so it can bond to clean surfaces. You still want to remove the top layer of existing grout to allow for the thickness of the new grout to finish flush with the surrounding grout. Cleaning the area with a tile and grout cleaner or vinegar and plastic scrub brush works well. Nothing sticks to old grout, not even epoxy, silicone, caulk or sealants.
