I've used a multi-step product to sort of epoxy coat the inside before the world wised up and put plastic tanks on dirt bikes - can't remember it's name - I'll look around

In the old days we'd slosh around a bunch of ball bearings inside with lots of soapy water till no more loose stuff could be flushed out.  Then dry (rinsing with alcohol or acetone helps).  You will never remove all the rust and the coatings just seal over it where it may continue to spread.  However, you will remove all the loose debric that will eventually clog up the fuel strainer or filter.  Can't remember what size bearings - obviously not critical but heavy enough to do the job.  Bearings don't add extra grit like sand blasting would.  Good luck!

My experience w/ a 2-part marine coating an old boat fuel tank was it eventually flaked off and clogged up the line. Now,
agitating small ball bearings (or bird shot) in solvent sounds good. Consider the paint if you invert it, but scrub as much of the entire inner surface as possible.
What ever you do buy a couple transparent fuel filters and keep one in the fuel line.
The fuel sock needs to be good to keep junk out of the petc0ck too.

Good luck.

Sorry, should have clarified a bit...... the purpose of the detergent-water is to suspend the flakes for easy flushing out with a garden hose.  Also it's a safety factor in case there are gas fumes lingering inside.  If you want a more radical clean you can first use a naval jelly solution in there; see:
http://www.shorechemical.com/Section%207%20Tech%20Sheets/naval_jelly.htm
but this stuff is corrosive so wear gloves (phosphoric acid I think).  Then the water is for flushing and neautralizing this.  Finally, an acetone or alcohol rinse blends with the residual water with a better evaporation rate for easier air drying.  Not essential tho.  Hope this helps.