Get 1 or 2 gallons of Acetone, pour it in, and swish it around. Acetone will dissolve the liner very quickly and you should be able to return to bare metal quickly and easily.
Before you try a new liner, make sure metal is VERY dry and pour in new liner, and rotate slowly, never allowing liner to "puddle" or "pool" and set up. A even, consistent thickness costing is what you want.
I pour in, hold tank upright, then work along the bottom, slightly tilted to one side, working forward. Then tilt to other side, working towards the back again. Then invert tank, and work the liner along the top, towards the filler neck.
Before you empty it, look inside and make sure you've gotten full coverage. If not, repeat rotation cycle. Make sure both petec drain holes are clear and the neck is also lined.