The extraordinary clarity of acrylic sheet makes it an excellent substrate for backlit signs, which is why it remains the product of choice.
Acrylic is typically painted in reverse on the second surface of the sheet. Acrylic provides tremendous protection from the degrading effects of UV light. A number of different paints are available for painting signs. Some of the more popular brands include:
- Spraylat Lacryl®. 400 Series is a popular product for painting plastic sign faces.
- Akzo Nobel Grip-Flex. Compatible with a wide range of rigid plastic sheet, such as acrylic, polycarbonate, ABS.
Paint must be thinned to the right viscosity for spray painting. Always use the reducers that the paint manufacturer makes for their paints. This will ensure chemical compatibility. Always thin paints according to the paint manufacturer’s recommendations. Some thinners can craze acrylic sheet. Given a choice between a mild thinner and one with strong solvents, such as toluene or benzene, always use the milder one. Finally, always test the paint and the acrylic sheet for compatibility.
Before painting, stir the paint well to disperse the pigment. This step is critical. Failure to thoroughly mix the ingredients can result in the paint not drying the properly. Thin the paint according to the manufacturer’s instructions, only using the reducers recommended and in the proper amounts. Adding too much reducer to the mix can reduce the exterior durability of the paint. If you have questions mixing the paint, stop and ask the manufacturer for direction, before going into production.
Using too much thinner in your paint can result in a variety of problems. An excessive amount of thinner can craze the sheet. Too much thinner can also result in shortening the outdoor durability of the paint job. Over thinning the paint also lowers its viscosity, which can result in too much paint laid down on the surface causing the paint to sag.
Sign makers use plastics so frequently and with such success that they rarely ponder the problems associated with them. Because various plastics are formulated and processed differently, each can have its own unique characteristics, requiring special consideration and treatment prior to painting. For this reason, before spray painting either acrylic or polycarbonate sheet, carefully read the manufacturer’s technical bulletin and follow their recommendations regarding surface preparation, approved solvents and paints, and spraying techniques. In thinning the paint, use the recommended solvents and never substitute with unapproved solvents.
Strong solvents in the paint subject the plastic sheet to a chemical stress. That chemical stress, in addition to the mechanical stresses that are subjected upon the sheet during fabrication, can greatly reduce its impact resistance, which can contribute to breakage.
Because solvents can craze acrylic sheet, use milder thinners rather than the stronger ones, such as toluene.
In the sign industry, two popular paint systems used in spray painting acrylic and polycarbonate sheets are Spraylat’s Lacryl® 400 Series and Akzo Nobel’s Grip-Flex®. Before using any paint system in production, you should always test it for adhesion to the substrate. Paint, which is not matched for compatibility with the substrate, often does not adhere well.
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. Then check for good adhesion, using the tape test. After burnishing an aggressive tape, such as 3M’s 610 tape, over the painted surface, quickly pull the tape and see if any paint comes off in the process. If the paint stays in place, you’re good to go.
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 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.
An uneven application of paint on a signface will stand out when a sign is illuminated. To ensure a uniform coverage of paint, spray several light coats, rather than laying the paint on thick. As you are spraying vary the direction of the passes from horizontal to vertical. You need to build the color up slowly, spraying a number of light coats of paint. Spraying heavy coats of paint often results in a splotchy appearance.
Some veteran painters recommend that you spray paint with some back lighting. This allows you the opportunity to check your work as you are painting to see whether or not you are spraying the paint evenly.
This technique also allows you to view the transmitted color, which tells you how the sign will appear when it is backlit. To achieve the desired color, you will need to spray many fine coats of paint. First paint top to bottom; then bottom to top. Next paint from right to left; then left to right. Between each coat you will need to wait about five minutes for the solvents to flash off.
Not much looks worse on a painted substrate than dirt or other residue on the sheet. To avoid this problem, always keep your work environment clean and, most importantly, thoroughly clean the sheet prior to painting. When the weather is hot and the humidity is low, you may consider dampening your shop floors to keep the dust down.
Some plastics, such as acrylic, pose virtually no problems at all. For these plastics, the only substrate preparation is to clean the surface with detergent and rinse with water.
Although many acrylic materials are scratch resistant, it is possible to scratch it, so be careful in handling and cleaning the sheet. Avoid using brushes, squeegees or other cleaning devices that could scratch the sheet’s surface. After washing the surface, rinse with clean water and dry with a soft cloth to prevent water spotting.
You can, of course, paint acrylic on the first surface of the sheet. If you do, you should spray a clear coating to protect the painted graphics. The clear coating will provide protection from abrasion during cleaning.
When painting on the second surface, after the graphics have been painted you can spray a thin layer of white mixed with clear. This layer performs two functions. First, it protects the painted graphics. And second, it diffuses the lighting in the sign box, minimizing any hot spots and helping disguise any pinholes.
- Set the air pressure of your spray gun as low as you can, while still obtaining the results that you desire. Don’t set the air pressure too low, though. Dusting the acrylic sheet results in a matte finish.
- Test, Don’t Guess. Before going into production, test the paint with the acrylic that you will be using.
- Hold your spray gun about 12” away from the surface of the sheet.
- Begin spraying before the lead edge of the sheet and continue until you pass the trailing edge of the sheet. Do not start or stop spraying with your spray gun positioned directly over the sheet.
- Keep the spray gun moving at a constant pace.