Polythene was discovered in 1933 by two developers at Imperial Chemical Industries in Great Britain. The plastic is durable, flexible and inexpensive to produce. It has replaced metal in many items such as outdoor storage tanks. It is also used for packing materials, plumbing pipes and automobile components. It does not hold paint well because it is resistant to chemical interaction. However, with proper preparation of the surface, you can paint polythene with some success.
Wash the polythene surface with warm, soapy water to remove all dirt, grease and other substances that could interfere with painting. Rinse it well and let it dry completely.
Scuff the polythene surface with a fine grit sandpaper so the paint will have something to grab onto. With the tack cloth, wipe away all the dust generated by the sanding.
Place plastic dust sheets in the work area and secure them with painters tape. If the item to be painted is small, cover the work area with old newspapers. Don old clothes and safety glasses.
Roll, brush or spray the polythene surface with the acrylic paint. Use acrylic because it is flexible even when dry, which will allow the polythene to expand and contract without tearing the paint. Apply a thin, even coat and let it dry completely.
Apply several more thin, even coats. Let each coat dry thoroughly before applying the next. Apply as many coats as you can afford if the polythene product is a large outdoor structure such as a shed.
Weathered polythene may not need sanding because of surface wear. If the surface is not glossy, it may only need cleaning before painting. Plan on repainting the polythene every year or so if it is exposed to weather.