This thing is EXCELLENT! We didn't want to mix chemicals and become junior chemists, so this was the answer. It is a long tube of clay-like material. You roll the blue coating into the white center stock with your bare hands [with a little water] until the color is even. Then put the material in the gap and smooth it out like plasticine. the color is permanent until it begins to settle; So make a plan before you start. Compared to other products, the price is about half.