Dommie tank

Then buy some phosphoric acid - easily available on ebay. At around 15-20% concentration the tank will be derusted in around a day. Rinse out with washing soda dissolved in hot water to orevrnt flash rusting.

Save the acid as it is amazing what else you find to derust. When ready to dispose if it, it is ok to pour it down a drain as it is used commercially for drain cleaning. 

Nb: if you spill it on concrete, rinse it off quickly as it eats the lime in it.

With regard to Phosphoric acid, it is NOT specifically  a de-rusting agent. Phosphoric acid works with rust residue to form a pre-paintwork coating. Prior to painting as much rust as possible is removed by whatever method you choose to use. The surface being worked on should be perfectly dry. Phosphoric acid is then applied over whatever surface rust remains and allowed to dry,  the acid reacts with the rust (Iron oxide) and produces a coating of Iron Phosphate this will dry to a black surface coating and can then be painted. As said, using this acid for de-rusting and then washing it off is wasting money and defeating the purpose of using it.             I offer this advice on the basis of work I used to do as an apprentice.  This work involved the manufacture of steel fabrications for the MoD and it was stated that all steel used in the construction should be treated in this way  prior to finish painting.

To expand in Ian's post. The iron phosphate has a similar density to steel so unlike the much lighter iron oxide forms a protective surface layer like aluminium oxide does on aluminium and alloys. Its not as good but better than nothing and will ward off rust for a long time under paint, less so inside a tank. However you first need to get rid of all the low density rust before phosphating the steel so the iron phosphate is directly on the steel, so here you can use acetic eg vinegar or citric acid eg lemon juice to derust or electrolysis. 

I once cleaned all the rust out of a Dommy tank by putting a few handfuls of 1/4 Whitworth  nuts and bolts in it, then lashing it to a shaping  machine whilst I made valve spindles on a nearby lathe. It worked really well but made a noise like an Ariel Arrow.

Mine had been sealed but not ethanol resistant and bits came of into petrol - not recommended. I put a wall paper stripper hose inside and then steamed cleaned it. This worked very effectively with anything sticking to walls of tank coming off in sheets which broke up and dropped through petrol feed. I noticed on Youtube a load of videos with vinegar and baking soda to clean rust etc..  

Thats a bit harsh !!!, I always  really fancied a Golden Arrow,  John Surtees had one he was tinkering with  and I got a ride on it.  Not impressed with the gearchange I was put off. Even more put off when Wozzy  from the Nightingale Mob  chased me (on an 88)  and fell off one trying to keep up round Chicken Farm bends . Still I gained a friend for life!!.  

Bad karma to hijack a posting about a cure for rusty Dommie tanks to postings about Ariel Arrows, etc

Mike