Homebrew PCBs

Gentlemen,
For a number of years I have been using my roll solder machine to tin my 
completed etched boards for soldering.

Part of the process involves removing the tarnish from the copper before 
applying the water based flux and passing the board through the rollers.

The tarnish remover was a Multicore solders PC2 Tarnish remover.

Disaster struck last week, the plastic bottle sprung a leak and four liters 
spread over the steel shelves and finally on to the cement.

Not a pretty sight. The shelves were stripped of paint and the cement 
surface etched away.

Apparently Multicore don't make this product in this country any more. So I 
need a replacement.

The label shows it contains Thiourea and dilute Hydrochloric acid. The 
Thiourea is 4%.

Anyone know what this Thiourea is and is there a better way to chemically 
remove the tarnish.

I will most likely have to make the mixture my self as  the brand names 
available in the USA are seldom available here.

Any comments welcomed

David