I just had this problem! Like discussed above, there is a hump (tunnel) between the two lobes of the tank, what you are seeing is bare metal and the green stuff is the liner. The only way to fix this is to completely remove the old liner and then you will need to re-seal it. Once it starts flaking/sluffing off it will keep going. Also the tank will eventually rust since the fuel we use now has some water in it (and ethanol attracts moisture too). While the petcock screen does a good job at big chunks the little bits will make it to the carb and clog it. When mine started it was so bad that it would completely plug the screen and the carb and it would stop running. It would also stop the flow of fuel completely intermittently and was a real pain to diagnose what was going on. Also agree that a good filter is a good idea and installed a Pingle in-line to help...especially with the fuel now a days.
There are a few good products out there and several good videos on how to remove it and reseal it on youtube. Word of advice, spend extra time protecting your paint, the chemicals you will use will find any defect or scratch and will take the paint off as well.