A closure device with interlocking fastening strips and a slider which establishes a leak proof seal. The closure device comprises first and second interlocking fastening strips arranged to be interlocked over a predetermined length and a slider with a separator finger which slidably engages said first and second fastening strips. The first fastening strip includes a first closure element and the second fastening strip includes a second closure element. The slider facilitates the occlusion of the closure elements when moved towards a first end of the fastening strips. The separator finger extends into the closure elements without extending completely through the closure elements. The portion of the closure elements below the separator finger are occluded when the slider is at the first end of the fastening strips.