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Decorating Defects - their cause and cure

Tarnishing of Metallic Paint

Paint made from metallic bronze powders will not retain its lustre for long when exposed out of doors, and soon becomes black under the action of air and moisture.
Most metallic paints are made from aluminium bronze powder which oxidises rapidly and takes on a dull hue. In industrial areas, where the air is often charged with impurities of an alkaline nature, these will attack the aluminium and tend to cause it to disintegrate.
If they are to retain their brightness on exterior work, such paints should be given a protective coating of varnish as soon as they have hardened.
A good exterior quality pale oil or synthetic-resin varnish can be used, although in the case of the latter there is some slight risk that it may have a solvent effect on the paint, if this has been mixed with an oil or oil-varnish medium; not all types of synthetic-resin varnishes will, however, act in this way and some may be used quite safely over old oil paints or finishes.
Gold paint should never be applied directly on to a metallic surface, or what is known as " electrolytic" action may be set up and darkening develop. A thin coat of shellac knotting should first be put on.