Painting Plastics

The most common paint for the plastics that can be painted is Acrylic Lacquer, when applied to acrylic sheet or polycarbonate a half strength thinner is generally required. Materials such as ABS and HIPS usually require a special plastic primer to be applied first, they also need a half strength thinner.

Cleaning 
Clean the sheet before painting to remove dust and ensure paint adherence. Since most plastics are sensitive to solvents such as aromatic hydrocarbons, concentrated alcohol, and ketones, use care in cleaning. Clean parts with a 25% solution of denatured alcohol and distilled water. However, for stains such as oil or grease, use a stronger cleaning agent such as kerosene. Be sure the sheet is fully dry and clean before painting.

Static Neutralization 
Before painting, it is important to neutralize electric or static charges that accumulate on the sheet surface. Dust on the sheet causes paint agglomeration and uneven layers. Since tearing the masking off the sheet will create a static charge, all pieces should be treated. 
One common way to do this is with an ionizing air gun. These guns safely and effectively neutralize electric charges. Alternate techniques, such as wiping the sheet with a damp, lint-free cloth or cleaning with a diluted alcohol-water solution, are also effective.
Avoid anti-static cleaners since they may leave a residue and cause paint adhesion problems.

Masking 
The protective masking is usually used as a protective layer for spray painting flat signs. However, many paint manufacturers also distribute liquid masking, which are commonly used to spray paint designs onto acrylic sheet. Drying time is usually about 2 hours, although it’s better to plan overnight drying to ensure complete evaporation. Drying can be accelerated by using forced-air heating at 40°C (110°F) do not go above this temperature as doing so may dry the surface and prevent evaporation of the water in the layers beneath. Leave the dried film in place until you’re ready to paint to prevent dust accumulation on the plastic surface. Just before painting, score the design on the film using an utility knife, giving it just enough pressure to cut the film without scratching the plastic. After painting, leave the film in place until the paint is thoroughly dry. Otherwise, you’ll get smeared paint and uneven edges. 

Painting 
Before painting, practice on a few test pieces to be sure that paint viscosity and air pressure are correct. Too high a delivery rate will result in too much paint and cause paint sag. It may also cause crazing due to too much solvent. Too low a delivery rate will result in “dry spray”, a matt surface caused by too much dusting. 
As a rule, use the lowest pressure at which you obtain correct results. Hold the gun about 300 mm (12") from the work piece, too close or too far will cause the above listed defects. Move the gun at an even pace and in a straight line.  Its movement should never start or stop directly on the sheet surface. 

Vary the direction of the spray, horizontally and vertically, to assure uniform coverage. Usually, four or five passes with several seconds between coats will provide sufficient paint. As mentioned, a light box behind the sheet will help in judging the uniformity and intensity of colour.

Paint Removal 
If you need to remove paint from the surface of a sheet, take it off immediately with the paint manufacturer’s recommended cleaner. Apply the remover using a rag; wipe off paint using a clean rag. Because paint removers contain organic solvents, minimize the time the remover is in contact with the plastic to reduce the chance of crazing.