The extraordinary clarity of polycarbonate makes it an excellent substrate for backlit signs. Generally, but not always, the sheet is sprayed on its second surface. Because the paint faces the inside of the sign cabinet, it is protected from the elements. Any painting done on the first surface of the sign face must be clear coated.
Paint Selection. In selecting a paint for painting polycarbonate sheet, follow the manufacturer’s recommendations. In thinning the painting, use the recommended solvents. Because solvents can craze or craze polycarbonate, use milder thinners.
As a rule of thumb, avoid using keytones, as well as chlorinated and aromatic solvents. Generally, paint systems based on aliphatic hydrocarbons, such as mineral spirits, alcohols, heptane and hextane, are compatible with polycarbonates.
Cumulative Stresses. Cracking and crazing are noticeable. But not all chemical damage is visible. Even if you cannot see a physical change to the sheet, the damage may already be done and can ultimately lead to failure when combined with mechanical stresses, both in fabrication and after the sign is installed. Stresses on plastic sheet are cumulative. One builds upon another, until you literally reach a breaking point.
Read & Heed. Before spray painting polycarbonate sheet, carefully read the manufacturer’s technical bulletin and follow their recommendations regarding surface preparation, approved solvents and paints, and spraying techniques. Two popular paint systems using in decorating polycarbonate are Lacryl® 400 made by the Spraylat Corporation and Grip-Flex® FR-2 from AKZO Coatings.
Test, Don’t Guess. Regardless of what type of substrate you are painting, before you use a new paint system, the best practice is to “TEST, DON’T GUESS”. Spray a few samples of the substrate with the paint system.
Adhesion Testing. After painting, check for good adhesion, using the tape test. After burnishing an aggressive tape,such as 3M Brand #600 clear tape, over the painted surface, quickly pull the tape and see if any paint comes off in the process.
Surface Preparation. Thorough cleaning to remove dust and other contaminants will ensure paint good adhesion. In prepping the surface, be sure to follow the specific recommendations of the paint manufacturer.
The right way to clean polycarbonate, is to first wipe the surface with a clean, damp rag or chamois. This will remove the static charge in the sheet and help clean any contamination from the sheet. The second step is to complete the cleaning, by wiping the substrate with a mixture of denatured or isopropyl alcohol and distilled water. The recommended ratio of alcohol to water varies from 25% alcohol and 75% distilled water to 50/50 alcohol and water.
Be very careful in using any solvent stronger than alcohol for cleaning. Strong solvents will subject plastic sheet to stresses that often result in crazing and cracking. If you need to clean a sheet, which has been stained by grease or oil, you can try a stronger solvent, such as naptha. If you must use strong solvent, allow the sheet to dry completely before painting.
Static. If you use the proper cleaning procedure, you should kill any static charge in the plastic. Like many other types of plastics, polycarbonate can build a static electric charge. Spray painting a surface with a static charge can result in a blotchy finish, which will stick out like the proverbial sore thumb, when the sign face is illuminated.
Sign makers use a couple different techniques to kill or neutralize the static charge. One common practice is to wipe the surface with a damp, lint-free rag. Others wipe the surface with an alcohol/water mixture.
Another practice is to kill the charge with an ionizing air gun. Do not use anti-static cleaners or dryer sheets, which can contaminant the substrate, potentially causing paint adhesion problems.
Spraying Technique. Spraying some test panels is also a good habit to get into, just to check whether the paint is thinned to the right viscosity or to check your air pressure settings are correct. As a general rule of thumb, you will want to use the lowest pressure possible, while still achieving the desired result.
In spray painting plastic sheet, you will want to apply very thin coatings. Applying heavy coatings, can result in either the paint sagging. Heavy applications of paint also result in a high concentration of solvent, which can craze the sheet.
Spraying a coating of paint, which is too thin, has its own set a problems. Dusting on the paint can result in a slight texture, which will give the paint finish a dull or matte appearance.
The technique that many professional painters use, when spraying, is to keep the nozzle of the spray gun about twelve inches from the surface.
In spraying, move the spray gun in a straight line from left to right (horizontally). You can also vary your spraying movement, by painting a coat vertically or from top to bottom. Varying the direction of the spraying motion, will ensure that you achieve a uniform coating of paint. Keep the rate at which you spray at a steady pace. Continue spraying until the pattern is off of the substrate. Never stop a spraying pass on the surface of the sheet.