I'm not sure what your gobby does to them, but one way you use putty is a) out of water, superglue the frag to a rock that's got some substance to it. Let it set enough. b) get a wad of putty, and stick it to the rock on the side that will be pinned down by another rock. Holding same carefully, because while fresh, it will fall off---lower frag into place, set the rocks, and hold them pressed that way until the putty has a little time to set. Once the putty has set up, it hardens in the shape of the frag's rock and the adjacent rocks, as mortar does around a brick. At that point it could be lifted off, but can't be shoved off sideways. HTH.