one shot drain cleaner - de-rusting

[Parazine]

Parazine and Bob, 
    what ratio of washing soda to water do you use?

My bath is a large plastic trug thingy from a garden centre. It takes about 8 gallons to fill it (that's about 36 litres for all you metrificated guys) and about half a packet of soda crystals (the small (500g?) size). I don't think the ratio is critical, it's there to make the water into a good conductor.