Bonderizing is basically zinc phosphate coating of the bare steel frame to give good anti-corrosion propeties, and improved the paint adhesion. David Rattray's the builders of the Flying Scot's used something they called the "Pyrene" process to apply it, which I assume was using a heated tank as described on this page ;
http://freespace.virgin.net/
Bob Reid Stonehaven Scotland (nowhere near Bristol and not enjoying an Indian summer unfortunately)
p.s. I think Henry Ford invented the "Bonderizing" name if you believe everything you read on the net.