Flooring Tile - Hairline cracks in new grout?

We layed 16x16 ceramic tile on a concrete floor and put grouting in about a week ago. Today I applied sealer (tefelon based) to the grout and small hair line cracks appeared across in some areas of the grout. Can anyone tell us what went wrong and do we have to remove the grout and start over? Thanks.

How wide are the grout lines?
Did you use sanded or unsanded grout?
How thick or thin did you mix the grout when you applied it?

Our grout lines are 1/2" and it is sanded grout (for 1/8-1/2" joints). It was mixed to a creamy paste.

1/2" is wide, making proper application critical. Perhaps you mixed the grout just a bit too wet. The wetter the grout, the more it will shrink when it dries. I usually use just enough water to get it damp.

You can either leave it alone, or use a grout saw to remove the top 1/8" and regrout those areas.

Wait a bit to get some other opinions. There are more knowledgeable folks here than I.

Eubie, 1/2 inch grout joint is a little wide for store bought sanded grout. For grout joints 1/2 inch and larger I usually add about a small coffee can of 20 or 30 grit silica sand per 25 lb bag of grout which will give your grout more strenght and reduce cracking. Having said that i would agree with John about cutting the joints or you may want to try a colored grout stain to match your exsisting grout in a inconspicuous area to see if it will fill in the cracks.

hope this helps:) Marco