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Usually one or two layers of protective lacquers (varnishes) are applied to one of the metal sheets sufaces while a decorative coating is applied to the other side. This coating is former with primer, printing inks and a finishing lacquer to protect the print.
Lacquers are applied as solutions or dispersions in an adequate solvent. Through the solvent evaporation and eventual chemicla reaction, the solutions become a solid film adhered to the metallic substrate. In general, lacquers and printing are applied to the metal plate before the tin container is manufactured, although they can also be applied to a finished container for certain use. The solution is applied to sheets previously cut to container dimensions until the final coating thickness is reached. The sheets are then dried in an oven where the solvent is removed and the varnish polymerizes to form a solid film.

Methods that can be used for lacquering:

a) Electrophoretic Lacquers

b) Dip Applied Lacquers

c) Brush Lacquers

d) Spray Lacquers

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a) Epoxy phenol

The most widely used at present.

Resistant to acids, with good flexibility, adhesion characteristics and high heat stability.

Widely used for fruits, fruit juices, vegetables. soup, meats, and marinated fish.

b) Acrylic

Used when a good color retention and high resistance to heat are required.

Formerly used only for exteriors. They are now provide internal protection as hard white coatings with an attractive ceramic and stain-free appearance.

c) Phenolic

Outstanding chemical stability and low permeability, especially against sulfide ions.

Used to contain meat and fish products.

d) Vinyl resin

Good adhesion and flexibility. It cannot resist sterilization at high temperatures. Free from odor and flavor.

Function as second lacquer layer for beer, wine and carbonated beverages and for cakes, sweets, dry foods and pharmaceutical products.

e) Oleoresinous group

It is usually modified with the addition of zinc oxide so as to prevent blackening of product due to reaction of thins with sulfur compounds.

Subdivided to two groups : “R” or fruit enamels – protect the natural pigment of highly colored fruits such as dark berries, red beats and red cabbage. “C” enamels – prevent the discoloration of foods containing proteins such as corn, peas, poultry and seafoods.

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a) Internal coating

Internal coating is coating that applied on the layer of can which is intact with food.

It provides protection of the food from the metals. For example, discolouration of some dark-coloured fruits such as plums and strawberries due to the metal contact.

It also provides protection from the food contents of the cans. For example, acidic carbonated drinks may c orrode the uncoated metal.

b) External coating

Needed to be lacquered at outside of the can if there is a poor storage condition to protect the can from corrosion.

External coating protects the can from the environment. It also used to support the decoration, labelling and consumer information on the can by providing a shiny, glossy and protective layer.

Influence mobility (friction) of the article during filling operations. Example: beverage cans can only be filled with an external decoration, which provides the necessary friction (mobility) to pass through the filling head.

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Lacquering seals and protects the metals from direct contact with the food. Thus, inhibit the chemical reactions such as corrosion and  food spoilage that will affect the food flavor.

It also prevents the dissolution of tins into the food products.

Different metals may require different coating systems. As examples, soft drink cans made from ETP (electrolytic tinplate) normally have a different coating to those made from aluminium. TFS (tin-free steel) food can ends need both internal and external coatings for protection, unlike ETP.

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Lacquer is a resin coating of cans which protect the inside of the can from corrosion and food spoilage.