Great picture eh? Night shot of my grouting the tiles around the shower. OK this is my exhausted thoughts on how to regrout tile:
1. Get up close and stare at the grout between your old tiles. If you see little spaces, tiny little missing pieces or cracks you need to replace/regrout. Doing this yourself is easy and better than the alternative which is having some men walk all around your place with muddy boots and break open the dry wall to replace it because you have water damage. As far as I can tell you should do anything you can to avoid water damage because that involves rot and that is expensive.
2. Buy the ready- made grout in the plastic can rather than the stuff you mix yourself from powder for the same reason you buy ready-made icing for a birthday cake when you are tired rather than make your own.
3. Take a pointy knife from the kitchen and scrape it along all your old grout to see what falls out. What falls out needs to be replaced.
4. Have a device for applying the new grout in between the old tiles. You can get tools for this, I used a the side of a bamboo point turner and it was just perfect. (I also slept with a seam ripper under my pillow when there were rumours of an escaped crazy person in the neighbourhood - trust what you know).
5. Smear the grout messy and diagonally so it gets in all the places over the spaces between the tiles just like you would ice a cake. Don't worry about getting stuff on the tiles because it can be cleaned off. Even if there is only one small piece missing in one line of grout do the whole thing around the tile. It's like fixing topstitching if you just do it only in the little place where there is a mistake it will look bitsy - best to stitch/grout one continuous line.
6. Read the instructions on the side of the plastic box about drying times and when you should wait to start wiping away the excess. This actually should be step one. Don't read this after you stop to have a 40 minute conversation on the phone with your mom because, just so you know, the window of opportunity for wiping away the messy extra is 15 minutes.
7. They say to use a damp sponge (the grout until it dries is water soluble) to wipe off the gunk from the tiles. I think they say sponge because that is all those grout men know and they don't want you to go digging in with something sharp. However if you have 400 micro fiber cloths because you got a deal on micro fiber fabric and cut it up and serged the edges because that seemed to be more interesting than actually cleaning the house, well you can use those too.
That's it. Takes a while but not hard and requires less brains than fusing interfacing.
Cost about $15.00.