Plastic needs special treatment, and that with regard to substrate preparation and paints to be used. You must degrease the surface with thinner and then with fine (from 180 grit) roughen sandpaper. In the past, special paints were required for plastic surfaces, you can now spread most acrylic and synthetic resin paints on plastic. conditions: The suitability is shown on the paint can and you use a special adhesion promoter as a primer before applying the paint.
With the metals, the whole thing is a little more differentiated. The usual paints are generally not suitable here, for iron- as for non-ferrous metals, you can find special paints at hardware stores. For all metal paints, the appropriate primers are always recommended, so the paint doesn't peel off.
When processing zinc or galvanized parts, you should not paint brand-new objects, but first weather them thoroughly, that means degreasing by wind and weather, dull and roughen.