rusty petrol tanks.

[pancho]

 In the current Scaysbrook mag OBA there is a "how to" on rust removal from inside a tank using electrolisis. Looks good.
  cheers pancho.

[cheapracer]

I use hydrochloric acid - otherwise known as swimming pool acid, takes 5 minutes.

Clean out thoroughly with water about 10 times then rinse a couple of times with metho spirits to remove the water then fill with your favorite fuel. 

[Stankflapper]

Phosphoric acid may be used as a "rust converter", throw a handfull of nuts and bolts in there and move the tank around to loosen the scale inside the tank, this is farrrrrrrrr safer.

[SimonC]

Where do you source phosphoric acid?

[LWC82PE]

I also use Hydrochloric acid from the hardware store. Sometimes they dont have it on display and you have to ask for it though. Why 'convert' the rust when you can remove it thoroughly for good in a short space of time.

[Freakshow]

do you water down the acid or use it neat

[LWC82PE]

Neat.

[cheapracer]

do you water down the acid or use it neat

I would suggest 50/50 and wear glasses just for your own safety if you haven't done it before but I use it straight. 

I just started to tip some into my pool one night and the first dollop somehow splashed straight back into my face and into my right eye and even immediately within seconds diving into the pool eyes open to flush it it was still a hospital trip so be careful.

By the way I also use it to clean aluminium but this is a process that takes around 1 minute, don't leave it in very long - ask me how I know!

Glad you asked, this acid is just awesome for leaving gearbox bits or other severly rusted up steel for hours and coming back to it finding it spotlessly clean. One time I bought an old Jawa gearbox at Gatton Swap Meet that was solidly rusted up so I left the lot in a bucket of acid over night - the next morning presented me with a bucket of black scum, a brand new gearset and not a single trace of aluminium to be seen!