Hi on another forum im hearing white vinegar if its surface rust.
or this stuff http://www.ftrs.com.au/redkote/red_kote_distributors.php
or http://www.kbs-coatings.com.au/kbs-motorcycle-tank-sealer-kit
or Put in some phosphoric acid and turn the tank around several times until its all coated, this will disolve all the crud and rust deposits. Leave it in there for while it won’t hurt. Then wash it all out (comes out like foam) with water and try to dry the tank quickly with a heat gun and compressed air. You can then flush with some Methalated Spirits as this causes the water to run out and will disperse it. Should be clean as a whistle after this.
All tis is copied from another forum i look at so if it melts the MX80 to the ground dont blame me hahahahaha…
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