I am lucky atm they have a blacking system where I currently work..
Have made some parts for drop targets .

 interested in the zinc plating system.
How many restores do you get out of the 20 litre system

Depends on how many parts need to be plated. A 20L Kit would last over 12 months. I've plated almost every Zinc plated part in a Dr Who, Joker Poker, Sky Jump, Free Fall, Addams Family (X2) and Indiana Jones. On top of this, 200 Legbolts, washers and other metal parts lying around including tools. If you prep the parts properly, the bath could last longer. The bath does lose the chemical ability to plate over time due to loss of water through filtering and heating. The bath can also perish if you contaminate the bath by plating the wrong parts, or leaving a part in the bath for too long.