Mr Davis,
Here is another chemical option. People use it for rusty vintage car and truck tanks.
The stuff is called Red-Kote. Some data 4 both types-liquid and powder (make your own liquid - better for posting. Have a look at the following site.
www.ftrs.com.au/redkote/red_kote_dry.php
Your nearest retailer would be Bob Stevens at Southern Districts Auto Electric Service, 20 Ironbark Ave, Camden South, NSW. 2570. Phone 02 46558243
I have never used this product and certainy do not know anyone connected with the business in Camden South.
I have to get some of this for my 1928 Chev truck fuel tank.
Stationary smile

I used to be indecisive. Now I'm not so sure.