I do grout professionally. If you have a large house, here are some fast helpful steps for a full redo ( adding important tips I learned the hard way.)
**TIP **using floor fans, and turning down the AC in your home reduces dry times and speeds up this process. do not use them until AFTER the cleaning is done, too much fan will dry the cleaner before you get to it. YOU NEVER want your cleaner to dry in the grout.
1. DO NOT USE BLEACH. it destroys the chemical bond of grout adhesive over time. DO NOT SEAL OVER DIRTY GROUT. its like painting a dirty house, its just going to peel off in a year or 2 because it has nothing to bond to except dirt. grout is porus, and for epoxy based sealers to sticj and perform correctly, the MUST soak into those dirt filled pores, so the nasty has to go!
**Use Zep grout and tile cleaner and whitener from home depot. it is by far the best overall product and does an incredible job. I would however suggest using gloves **DANGER!!!** if you have marble or granite or stainless steel appliances, KEEP THEM AWAY or cover them, this product will ruin them.
2. squirt product in grout and allow to foam. give 2-5 min. when 1st learning, try to stick to small 6x6 foot sections.
3. scrub grout with a hard nylon brush. then sprinkle a little water and scrub the tile.
4. dilute floor with 1-2 cups of water and swish around with brush or broom on a stick. then suck up all dirty water, with a shop vac or wet vac and dry remaing water on floor with towel. repeat on remainder of flooring.
5. allow grout to dry for at least 2 hours, i prefer to clean 1 day then dye the next
6 If in expensive home, use painters tape on all baseboards right at grout line.
i use JAMO brand dye. they have the best selection of colors, and a superior product. it comes in 1 quart cans and can be purchased or ordered at most tile stores
7. Use 2 inch brushes to paint dye into grout. do not worry about what gets on tile. as before, do smaller sections at first. SPREAD EVENLY!
allow product to dry for about 15-25 min, until it sticks to tile, but can be easily peeled off by fingernail.
NOTE step 8 requires a floor buffer, or on small jobs a piece of abrasive hand size padding ( sold in same area of home depot buffers and pads are rented or sold)
8. after 15-25 min, lightly sprinkle water on the tile. Turn on floor buffer, slowly adding water until you see the dye start to disintegrate into the water turning it a milky color. do several passes in both directions, let the buffer do the work for you.
9. Again, use the shop vac/wet vac, to suck up your residue
10. touch up smaller areas you missed with a hand pad. wait for 1 day dry on any areas stain did not hold, the reapply and remove from tile.
**you will notice in larger tile homes you quickly learn to do larger sections at once.
What will be left is a floor you will love again, with a SUPERIOR stain resistance. most legitimate color sealers offer a warranty, and teach self repair for small "unforeseen" homeowner accidents.
make sure if you hire someone they leave the remainder of a can, you may need it in the future, even if just for the color information.
I have spent 10 years in norther brevard FL perfecting this skill and have repeated it in hundreds of homes, and charge and average of ###-###-####$ for this work- and I just gave the recipe to you for free, ENJOY!
yourteamcondo.com feel free to call my company anytime for free phone info if you get stuck or have questions.