As long as the original grout is less than 48 hours old, it should bond fine. Mongo gave you one valid argument for the blotchiness in the grout (the need to fully dry first). There is another, though, which WOULD be helped by skim coating the grout, and this problem is more prevalent with white grout, especially with white tile, than any other because of the contrast. What you may be seeing is the color of the sand in the grout. If it wasn't properly mixed to start with, he may have clumps that contain more sand than the rest of the grout, and as a result, it would show as dark spots in the grout, with a tannish tinge (which is what made me think of this).

Hi Ginny20 and Bethcw. My name is Ken Sherman, President of This Old Grout. I was alerted to your posts and wanted to let you know that our Color Seal product works really well to solve the grout challenges you have. Color Seal will make the grout completely uniform in color and stain proof the grout for years to come. The longevity of the protection is 10-15 years. Our website has videos explaining the application of the Color Seal...it's very easy to use and environmentally friendly; you won't need harsh cleaners to keep your grout looking brand new. For more information, feel free to call us toll-free or review our website at www.thisoldgrout.com. Thanks, Ken P.S. Thank you for your kind words gbsim...always great to hear from our satisfied customers. :) Here is a link that might be useful: This Old Grout Color Seal

The caulking should be where the tile butts up to the window frame. one thing you might consider is using a piece of travertine slab for the shelf.

"The GC is telling me the cracking will soon stop (in other words, within the first year) -- is that more BS or is it true?" Maybe yes, maybe no. In lightweight frame construction, the framing lumber can take several months to a year to come to an equilibrium of sorts. During that time, the wood can move. Moreso if the framing lumber was green (not pressure treated green, but non kiln-dried green) instead of kiln dried. Still...houses move. There is still seasonal movement due to humidity, subtle movement from wind loading, etc, etc. So while grout in changes of plane is more likely to crack within months of the initial installation, it could crack year after year. Or after every other year. Or never again. There's just not a one answer fits all solution. Cracked grout in decorative tile is cosmetic cracking. But cracked grout in a wet area like a shower represents another avenue for liquid water and water vapor to get behind the tile. Normally not a problem with a topical membrane like Kerdi or RedGuard. But still, industry standard, and best building practices, dictate that with inside corners in wet areas, it's best to use color and texture matched caulk instead of grout. Caulk flexes. Grout doesn't. If their caulk is failing, then they need educate themselves on how to properly caulk, prep, application and tooling, and on using the proper caulk. Mongo