Why Paint Opacity Matters

Here is why understanding paint opacity makes a difference:

1. Number of Coats: High opacity paints contain opacifying extenders and high refractive index pigments that block the light from penetrating to the substrate with fewer coats. It seals the surface and does not smudge in the surroundings. 
2. Paint Performance and Finish: High opacity paints provide a glossy and satin finish without bumps. Low opacity paints ruin the overall appearance due to frequent coating and cracks. Adjust the opacity of paint to achieve uniformity in the finish.
3. Hides Imperfections: High opacity paints allow you to paint lighter paint over the darker ones. It hides the dents, cracks, and uneven surface with a smooth finished look. Use the right application method and tools to minimize brushstrokes and marks behind.
4. Saves Time and Cost: Paints with high refractive index, like enamel or alkyd paints, require less labor and amount of paint to color the substrate. They reduce the paint wastage and the need to re-purchase. Unlike low-opacity paints that follow the coat-dry process, high-opacity paints require shorter project time.

What are the Factors That Affect Paint Opacity?

Here are the five factors that affect the paint opacity:

1. Type and Quality of Pigments: There are three types of pigments, including opaque, transparent, and semi-transparent. Pigments with a high refractive index bend the light effectively. Titanium dioxide (TiO2) has a high RI, and its white pigment seals the substrate. TiO2 contains high opacity rutile and anatase forms. Rutile has the highest refractive index of 2.7-2.9, and anatase has 2.5-2.6. Zinc sulfide (ZnS) is also a high opacity white pigment with a RI (n) of 2.35 at 500 nm. Unlike titanium oxide, Zinc oxide (ZnO) has a low refractive index of 1.9. Adjust the concentration of the right pigment to achieve the desired results. 
2. Paint Type: Flat and matte paints absorb the light to provide a sheen and high opacity finish. Glossy paints have less hiding power due to lower pigment volume concentration (PVC). 
3. Paint Color: The color of the paint also affects the paint’s opacity. Darker colors like black, red, blue, and green have high opacity and sealing power. Whereas the quinacridone, phthalo, lemon yellow, and pyrrole red are transparent colors. 
4. Binder and Resin Content: Paints with emulsion paint binders and resin contents form a thick film to disperse light. Opacity of paint binders also depends on their interaction with pigments. For example, linseed oil changes its opacity in contact with oil-based pigments.  
5. Application Technique: The technique used to apply the paint also affects its opacity. Brushes often leave brushstrokes and minor marks, which reduce the paint’s opacity. Applying paint with rollers increases opacity as they cover and spread the paint evenly. Spray and airbrushes reduce the paint opacity due to lower paint thickness and faster speed. Choose the right tool and high opacity paint to reduce this stress. 

How to Choose the Right Opacity for Your Project

Choose the opacity according to the area that needs to be painted:

1. High Opacity: Prefer high opacity paints when changing the wall colors, hiding the ink stains, and running out of time. They are used on new plaster, uneven, porous, and lighter surfaces. 
2. Medium Opacity: Use medium opacity paints for glazing art, staining wood, and restoring the same tone walls. 
3. Low Opacity: Low opacity paints suits best for layering decorative items, highlighting textures and for water color effects. They are used on aged furniture and glass panels.

How Can You Improve Paint Opacity for Better Coverage?

Make sure to prime the surface well before paint application to achieve a high opacity finish. Read the instructions of the manufacturer and use the right application technique. Apply paint in different directions to avoid bubbles and air traps. Mix the additives like white pigments (TiO2), carrier mediums (Liquitex Titanium White), and thickeners (acrylic, polyurethane, cellulose) to increase the opacity. Apply 2 to 3 coats of thinner paints and wait until the first one dries completely before applying a second coat. Select paints with a high pigment and binder ratio (oil-based paint) for better coverage. 

Standard Test Methods to Measure Paint Opacity

Here are seven standard tests that are used to measure the paint opacity:

1. ASTM D2805 – 11(2018): A liquid paint is applied on a test card to measure the hiding ability of the paint. This standard method utilizes reflectometry to examine results. 
2. ASTM D6441 – 05(2016): Like the standard method of 2018, this test measures the stain covering ability of powder coatings. 
3. ASTM D6762 – 18: In this standard test, a human examines the hiding ability of spray coatings on automotive surfaces. 
4. ASTM D5007 – 99(2017): This test is used to measure how well a paint hides when wet and after drying. 
5. ASTM D5150 – 92(2017): It is used to examine the ability of architectural paint to hide when applied with different tools.
6. ISO 6504-3:2019: This test assesses the ability of paint to cover the stains on rough surfaces.
7. BS 3900-D4:2006: It is used to compare the concealing power of two paints of different brands.