File:Jolli Joe all metal Banjolele back with reso.jpg

English: View from the rear of a "Jolli Joe", a budget pre WWII UK banjolele made of 'Zamac', a family of alloys with a base of zinc with added aluminium, magnesium and copper in varying proportions; Aluminium content is constant at 4%. Zamac was developed by The New Jersey Zinc Company in c.1929. They appear to be painted with a 'wrinkle' paint finish, that is often seen on old electrical component boxes or other utility metalwork. This wrinkle effect is achieved by applying a thickish layer of an alkyd-resin based paint, which remains relatively soft for some time during the drying process. A layer of quick-drying cellulose lacquer is then applied on top while the paint is still soft and as the lacquer dries and shrinks the wrinkling effect occurs. It would appear they were spray-painted, judging by the 'overspray' traces on the inside surfaces of the body. Flaking paint is also a common feature of these instruments and could be due to insufficient pre-treatments or to micro-corrosion of the alloy surface causing localised surface instability. Other causes are possible and long term paint stability was probably not considered a priority for these economy instruments. They are normally a denim-blue colour, but sometimes they appear green. The 'green' appearance is due to yellowing of the cellulose lacquer over time. The yellowed lacquer over the blue paint gives the illusion of green.