[jim sewell]

Internal abrasive method .
A method of doing it yourself is to make a frame that can grip the ends of the tank and rotate the tank in variable positions ie, horizontal or vertical and in between positions .
Put in some sharp stones ie blue metal ( aust term ) and some water and detergeant and rotate with a small elect motor or similar device ( remove the fuel gauge sender unit first ).
.
This should scrape away the surface rust etc
Regards
Jim S.