DIY. Sluice it out as best you can then let it sit for 24/48hrs full of a weak phosphoric acid solution. £20 for 5 litres..... Google it!
Don't know if this is helpful and might not be what you're looking for but... I had probs with the tanks from my two 90's Hondas. Basically they were rusting. Particles were flaking off, clogging filters and buggering up carbs. I tried de-rusting, using a product mentioned in one of the monthly comics. I'm pretty sure I followed the instructions carefully but it didn't work. It was bloody difficult trying to dry out the tank at the end because of the convoluted baffle system inside. I had them sitting over an oil-filled rad for 3 days! In the end I had the tanks lined. There are DIY kits out there but after my de-rusting experience I let the professionals do it. I went to biketankrepairs.co.uk. They're in Devon and the main man is Kevin Newton. So far everything has been fine with no trace of rust.
