silicon will soak down into the porus grout and fill up the cracks. You might and should do two or three applications of the stuff to work properly. do not use on old grout, I have found this to work better on new grout only.
apply a coating and wait for a while, then apply another coating. Then after a short drying time. get a teaspoon with water and drop a drop on the grout. It should bead up like water on a freshly waxed car hood. If not, apply more silicon until this occurs.
Once you have done this, it won't work after grease, grime and food are on the counter. You will then seal in old grime, no good.