petrol tank cleaning

[RDfella]

Hi guys.
Am currently working on the petrol tank for the project ('62 GF tank I believe). Have cut out the bottom to a) remove a dent and b) create a recess for the front carburettor. Wanted to do the welding whilst it was dry and vapour-free (has been standing unused for 30 yrs). My next job is to clean the inside. Rust is very light, but it's coated with a varnish-like substance from old dried-out petrol. What have members found the best for removing this? Cellulose thinners? Drain cleaner? Seems to me something should dissolve it fairly easily.

[Del 60]

I have had good results cleaning bike tanks with Apple cider vinegar, cheap to buy if brought from a horse feed outlet £12 / gallon

[Greybeard]

Cider will probably remove light rust but it sounds like RDfella has a petrol based residue. I think cellulose thinners would shift it.