How to seal a thin crack in the counter top of the kitchen

If the crack is less than the thickness of 3 sheets of paper, try the product from coriartinc.com, 800-724-4008 and use these directions. If the crack is bigger, or this is too difficult for you, maybe our expert (ID odinn?) will be along soon.

Lay a bag of ice across the crack for about 45 minutes. Make sure it lays about 6 inches on either side of the crack. This will widen the crack somewhat.
Clean crack with a toothbrush and hydrogen peroxide (3%). Wipe off excess. If dirt still seams to be visible, you can use a chlorinated cleaner. Making sure there are no open flames in the area, rinse with rubbing alcohol. Allow to dry completely.
Draw a pencil line along the crack about 1/16" away. In the next step the crack may disappear and you will want to know where it is.
Cover anything under the crack with a paper towel. The adhesive may flow through the crack and fall upon anything under it.
Apply heat, with a hair dryer, to an area 6 to 8 inches on either side of the crack. Heat the area evenly for about 20 minutes or until the crack disappears.
If the crack is not aligned vertically, you may want to apply pressure to make it flat.
Apply a very thin bead of Art Specialties International, Inc. Super Thin CA (tinted if this is a solid light color) over the crack while it is closed. It is important that the seam be completely closed. This industrial grade CA Will not properly wick or bond in a joint that is open.
Wick off excess adhesive with a paper towel. It would be wise to fold it twice in order to not glue your hand to the towel.
Leave the entire countertop alone for 24 hours. The growing strength of the bond will be stopped by any sudden shock. This would leave the strength of the bond compromised at less than 100%.
Hand sand off excess with 220 grit sand paper. Continue sanding to eliminate the warn edges of the crack. About .002" (1/2 the thickness of a sheet of paper) should be sufficient. Use waterproof variety wet to avoid dust. Then sand with 400 grit sand paper to approximate the sheen of the countertop. Buff out with a gray Scotch-Brite®
It may be a good time to renew the entire surface by wet sanding the whole top to the desired sheen. I like to use a wallpaper sponge as a backer to support the sandpaper.

Great post Joypulv!