Sandblasting is a way!
I've made it with TDM gas tank and result was great, brand new gas tank (unfortunately just inside
). First you screw out fuel tap, put the tube from sandblaster inside and leave leave it for a few minutes, then you turn of sand and make the same with just air from compressor, it will look good!
Don't forget to put of fuel
! Serious, wait befor all gas will go out, because send when it hit the tank will put it on fire in other case. Wash it then just to be shure, there is no send (20 minutes for all!!!)