Originally Posted by Pomo
I don't know if I'd classify it as it "strictly" cosmetic, it may indeed weaken the fuel tank. But the cracks will not instantly cause fuel leaks.
I don't have a cutaway, but here are pictures of where the fuel pump and filler neck are mounted.
After seeing these pictures, the tank is a one piece design. The white & black that you are seeing are merely different layers of the tank which is very likely blow molded. The white is likely going to be virgin material with no color added, the black is likely virgin and reground material with color added. If the tank design/fab is similar to the tanks that I worked with at a fuel tank manufacturer will be multiple layers of material with a layer of less permeable material sandwiched between the inner and outer layers of material.
If the tank is of the construction I am describing, there is no separation between the white & black material shown in the picture.
*former engineer at a fuel tank manufacturer