Painting Acrylic Sheet
The superior clarity and long-term durability of acrylic sheet allow for easy painting and printing of fabricated signs, displays and other decorative items. However, simply choosing to use the material is not enough to guarantee success. With that in mind, the following guidelines and the correct supplies will help you produce high-quality acrylic sheet products.

Paint

A variety of paints specifically designed for acrylics are commercially available. Most are supplied as concentrates and require thinning to obtain the correct viscosity.

As a rule, use mild thinner consisting of an alcohol blend, rather than one containing aromatic hydrocarbons, such as benzene and toluene. Always follow the paint suppliers’ recommendations about the amount and type of thinner.

Acrylic sheet’s extraordinary transparency allows for back-painting (i.e. applying paint to the side of the material not exposed to weather). The unpainted outer surface of the sheet provides weather protection for the paint. If you must paint the outer surface, apply a suitable protective coating over the paint.

Spraying Systems

Use any efficient, atomizing spray gun system that uniformly distributes paint. Consult your suppliers regarding the equipment you need for a particular application.

To obtain enough atomizing air, the available line pressure should be in the range of 414 to 621 kilopascals (60 to 90 psig). Use a 7.6-m (25-ft) long, minimum 8-mm (0.3- in.) diameter hose to avoid pressure drops in the line.

The air compressor should be large enough to provide the volume and pressure required for all spray guns.

Metal screen strainers at the pressure tank and spray gun will ensure equipment cleanliness, which is also crucial to uniform paint dispersion. Clean these items and the spray gun daily.

Install an adequately vented spray booth that is large enough to house your biggest work piece. Use a light box to judge paint uniformity on transparent plastics. Be sure the light box conforms to building code regulations for a spray painting area.

Fabrication Techniques

Fabricating acrylic sheet using incorrect techniques prior to painting may cause stress in the material. This can result in crazing (i.e. numerous tiny cracks) after painting. You can eliminate crazing by following the manufacturers’ recommendations for fabricating acrylic sheet.

Cleaning

Always clean acrylic sheet before painting. This will remove dust and assure paint adherence.

Use special care in cleaning, as acrylic sheet is sensitive to solvents, such as aromatic hydrocarbons, concentrated alcohols and ketones. Clean parts with a solution of distilled water and 25 percent denatured alcohol.

For stains such as those caused by oil or grease, use stronger cleaning agents, like hexane, aliphatic naphtha or kerosene. Make sure the sheet is fully dry before painting.

Static Neutralization

It is important to neutralize electric or static charges that can accumulate on the sheet’s surface and attract dust before painting. Dust on the sheet causes paint agglomeration and uneven layers.

Since tearing the masking off the sheet will create a static charge, all acrylic pieces should be treated with an ionizing air gun, which safely and effectively neutralizes electric charges. Wiping the sheet with a damp, lint-free cloth or cleaning with a diluted alcohol-water solution is also effective.

Avoid anti-static cleaners. They can leave a residue and cause paint adhesion problems.

Maskants

Acrylic sheet’s paper masking is commonly used as a protective layer while spray painting flat signs. However, paint manufacturers also distribute liquid maskants, which are often used to spray designs onto acrylic sheet.

Maskant is supplied as a thick liquid, consisting of water-soluble latex resins in a solution. It is applied through the use of air (or airless) atomizing spray equipment. Being a water solution, it must be stored at above 0º C (32° F), to prevent freezing.

Clogging may occur, due to dried film mixed with the solution. You can prevent this problem with a special nozzle— available from paint equipment suppliers—that reverses the flow and cleans out the buildup.

Spray the maskant on evenly to a wet film thickness of 250 to 300 microns (10 to 12 mils). It will dry to above 100 to 125 microns (4 to 5 mils).

Maskant Drying

Drying time is usually about two hours. However, it is better to plan for overnight drying to assure complete evaporation.

Using forced-air heating at 43° C (110° F) can accelerate the drying process. Higher temperatures might dry the surface and prevent evaporation of the water in the layers beneath it.

To prevent dust accumulation on the plastic surface, leave the dried maskant in place just prior to painting. Score the design on the film, using an exacto knife, applying just enough pressure to cut the film without scratching the plastic beneath.

After painting, leave the film in place until the paint is thoroughly dry. Otherwise, you will end up with smeared paint and uneven edges.

Painting

Before painting, practice on a few test pieces of acrylic sheet to ensure paint viscosity and air pressure are correct. As a rule, use the lowest possible pressure to obtain the correct results.

Too high a delivery rate will result in an excess of paint and cause paint sag. It can also cause crazing of the sheet, due to too much solvent. Too low a delivery rate, on the other hand, will result in dry spray—i.e. a matte surface caused by too much dusting.

Hold the gun 300 to 350 mm (12 to 14 in.) from the work piece; being too close or too far away will cause the defects listed above (Figure 1).

Move the gun at an even pace and in a straight line. Its movements should never start or stop when directly facing the sheet surface.

Vary the direction of the spray, both horizontally and vertically, to assure uniform coverage. Four or five passes with several seconds between coats will provide sufficient paint. Using a light box behind the acrylic sheet will help you judge the uniformity and intensity of color.

Screenprinting

When used for high-volume production, screenprinting is fast and economical. For beginners, it is best to purchase a screen from a local screen supply house.

After set-up, apply paint with the squeegee in a uniform and even motion in one direction. The paint will pass through the open screen mesh, transferring the pattern onto the acrylic.

The most important factors are the paint’s viscosity and the size of the mesh openings. These will determine paint flow through the screen and the paint’s appearance on the acrylic.

It is difficult to generalize what these conditions should be. Many different fabrics are used for screens and paint viscosity depends on the application methods and temperature. Consult your paint manufacturer for advice about thinning paint.

Paint Removal

If paint removal from the acrylic sheet’s surface is necessary, it should be performed immediately. Use the paint manufacturer’s recommended cleaner. Apply the paint remover with a rag, then wipe off the paint using another clean rag.

Paint removers contain organic solvents. Therefore, to reduce the chance of crazing, you should minimize the period during which the remover is in contact with the acrylic sheet.

Troubleshooting Guide

Problem	Cause	Solution
Maskants
Weak and brittle maskant	Air bubbles in film	Dilute slightly
	Film not thoroughly dry	Wait recommended drying time
	Maskant film too thin	Increase thickness to 3-5 mils (10-12 mils wet).
Too much adhesion	Maskant film too thin	Increase thickness to 3-5 mils (10-12 mils wet).
	Coating exposed to UV	Do not store faces outside.
Spray Paints
Poor adhesion	Incorrect paint	Use paints recommended for use with acrylic
	Dirt or residue on sheet	Clean sheets thoroughly before painting
Blotches of paint	Static electricity	Neutralize charges with ionizing gun or wipe with damp cloth
	Uneven paint application	Apply paint in more passes, using less paint per pass
	Paint not applied uniformly	Use backlighting to check paint as it is being applied.
Screen Paints
Poor detail	Screen mesh too coarse	Use a finer mesh screen.
	Paint too thin	Use less thinner
	Worn screen	Replace screen.
Paint drying on screen	Hot, dry weather.	Add retarder to slow paint drying.
	Too much time between screening	Flood screen between passes.
Crazing	Stress from fabrication.	Review fabrication techniques.
	Flame polishing	Flame polish as last step.

Notes:

Acrylic sheet is a combustible thermoplastic. Precautions should be taken to protect this material from flames and high heat sources.
The information and statements contained herein are believed to be reliable, but are not to be taken as warranty or representation for which CYRO assumes legal responsibility, nor as permission, inducement or recommendation to practise any patented invention without a licence.
Users should undertake sufficient verification and testing to determine the suitability for their own particular purposes. Be sure to follow manufacturers’ safety recommendations for equipment and materials used with acrylic sheet.

Written by Grant LaFontaine, Sheet Products Technical Service Manager for CYRO Industries.