There are few if any limitations to the types of wet paint that can be applied to zinc diecastings. For the best adhesion of any type of paint it should be applied after pre-treating the casting with a phosphate or chromate based passivation finish or an etch primer coat. The table below gives some examples of products and their use:
Commonly Used Paints
|Product||Condition of Use||Coating Thickness||Mode of Application|
|Etch Primer||Adhesion of second coat||<10μm||Air|
|Epoxy||Anticorrosion, excellent adhesion||>25 μm||Air
|Polyurethane||Excellent outdoor life. Good resistance to chemicals.
|>20 <25 μm||Air
|Acrylic/Polyurethane||Single coat colour finish||Air
Design of Castings to be Painted
The rules governing good design for economical and effective painting and lacquering are basically the same as those applying to design for electro-plating, though the results of ignoring them may be different. Thus a sharp edge tends to pickup only a very thin paint film whereas it attracts a heavy bead of electrodeposited metal. Holes and narrow recesses are difficult to coat by spray painting and may carry an excess of paint if the articles are dipped. Large, unrelieved flat surfaces tend to accentuate any faults in paint application.
Components are generally degreased and phosphated, when they can be painted with the same paints as the steel on which they are mounted, unless a contrasting colour is required. Hardware items are finished similarly, and so are some toys, while others are simply degreased and painted with no chemical pretreatment to improve adhesion. In the latter case acrylic paints containing an acid-etching ingredient should be used.
Instead of phosphate treatment before painting, chromating can be used and is recommended when better corrosion protection is desired, for example on articles exposed at the coast or at sea. In this case, stoving should not exceed 1 hour at 150oC to avoid dehydrating the chromate film. Alternatively, an etch-primer can be used instead of either type of chemical pretreatment, provided that the maker’s recommendations on subsequent painting are carefully observed.
Where the very best corrosion resistance is essential, it is recommended to specify paints based on epoxy-phenolic resins or epoxy amines, which are stoved at temperatures above 177oC. The epoxy-phenolics are yellowish and so are unsuitable for white finishes. Paints based on epoxy esters stove at lower temperatures and are less resistant to water contact. However, they are more corrosion-resistant than alkyd enamels, for example. Epoxy primers may be used on castings to improve the adhesion of alkyd top coats where the latter have been selected for other matching components of the same products.
If a one-coat paint finish is all that is required, it is necessary to specify a paint with good water-resistance to guard against early failure by loss of adhesion. For this sort of finish, an acrylic paint is recommended. Paints can be formulated with an acid-etching ingredient to help adhesion if the castings are only to be degreased, before painting.
Painting Over Chromium Plate
Attractive combinations of painted and plated surfaces are possible on zinc diecastings and special effects can be created by filling in plated texture or embossed surfaces. The diecastings are chromium plated all over, rinsed with deionised water and dried before being spray-painted through a close fitting mask. Minimum delay – not more than a few hours between plating and painting – is essential if paint adhesion is to be good.
Other Applied Finishes
Various chemical treatments are available for producing black or brown finishes on zinc alloy and some have found applications, e.g. for shoe heels and parts of cheap toys. To withstand frequent handling, the finishes need a final lacquer coating, and for most purposes better results are achieved by using paint in the first place. It is possible to dye chromate films on zinc alloy to produce a range of colours, but colour matching from batch to batch is difficult and fade-resistance is sometimes poor, so the treatments have met with little commercial success. A simple way to make a black finish is by immersion in ammonium molybdate solution.