You only should fuse an object one time, not scan 1 with scan 2 and then this result with scan 3 and so on!
Align all the done single scans, like here the head of a man:http://wiki.david-3d.com/david3_user_manual/overview
….and then fuse all to one object together. Forget the single textures, DAVID will handle them in the right way. The fused object comes as a „large“ OBJ file and the texture will be one PNG file where all the single textures are placed side by side. Like here the scan of a bulb (Sept 24, 2009) :viewtopic.php?p=8326#p8326
PS. Just before you start the fusion let DAVID do in Shape Fusion a final 'Global Fine Registration'. Then all the single scans will be optimal aligned one to each other.