Acheiving Desirable Hiding or Coverage

In this Newsletter we focus on routes to achieving hiding in latex Paints. By hiding or coverage we are referring to the ability of a coating to hide the substrate. For further grounding note that most paint cans today promise coverage on the order of 200 to 450 ft2/gal. Clearly, this is a useful number for estimating the amount of paint to complete a given job. (see Paint Calculator). The degree of coverage achieved depends on many things including the smoothness and porosity of the surface being painted. But there are other factors contributing to hide. In this newsletter we will focus on how the ingredients or paint formulation, as well as the application methods and tools, can affect hide.

A neat latex or emulsion generally dries to a clear, transparent film. At the other end of the spectrum are fully formulated paints and opaque stains which are intended to completely cover and obscure the surface on which they are applied. So why do paints have different spread rates and different coverage?

We think of the paint can as containing 4 main ingredients – liquid or carrier component; pigment; binder; and additives. The bulk hiding properties are controlled primarily by the pigment content, with the additives providing some ability to enhance and tune the final coverage.

The substrate appears opaque or the coating hides the substrate because light is prevented from striking the substrate. This happens because light is reflected, absorbed or bent (and does not make it back to the observer from the substrate). Everything else being equal, the pigments with the higher refractive index scatter light and are better hiding agents than the pigments with lower refractive index. As a result titanium dioxide (refractive index = 2.7) is a superior pigment to extenders like clay (refrac-tive index = 1.6) and diatomaceous earth (refractive index = 1.4)

Primary Pigments: Formulators routinely talk about primary pigments and extenders. Both white and colored pigments are regularly used today. In fact, we see a clear trend to brighter colors in the market place today. The primary white pigment in the paint can is titanium dioxide. It hides because it very efficiently scatters visible light. These discrete particles are engineered to the optimum particle size to reflect and scatter visible light. In principle, a higher the level of titanium dioxide would result in more particles scattering light and hence results in increase hiding or coverage. It is this fact that light does not strike the surface that results in good coverage and in the case of titanium dioxide results in the appearance of a white coating.

Colored Pigments: A desired color is achieved by combining the titanium dioxide (the primary white pigment) with colored pigments. Unlike titanium dioxide colored pigments tend to hide by adsorbing light rather than reflecting and scattering the light. Also some colored pigments are organic in nature and not particulate. As a result some organic reds give relatively poor hide. In other words, multiple coats of paint have to be applied to achieve the desired finish. Note that this is an over simplification – the way pigments and colorants work to achieve desired hues and tints is too complicated to be captured here.

Role of Additives: Rheology Modifiers play a key role here by primarily controlling the smoothness and thickness of the paint film. The more evenly the coating spreads, the better it hides. And the thicker the coat, the more likely that light would be scattered, resulting in an opaque finish.

Achieving Optimum Hide in a quality paint: It is not uncommon today to see paints claiming improved hiding or coverage in fewer coats. Formulators have found clever ways to optimize pigment levels, and use the right type of rheology modifiers and other additives to make products that hide better. And to overcome the limiting physics of efficiency with some colored pigments, manufacturers now recommend the use of tinted primers with some systems.

Don't be fooled by the "false hiding" ability of some low quality white or light-colored interior flat wall paints. These paints use high levels of "extenders," such as clay and calcium carbonate. Their powdery finish may initially appear to have thoroughly covered the surface. However, they lose much of their hiding if the surface gets wet.

See below for further guidelines on prudent steps to maximize hiding and coverage.

ACHIEVING COMPLETE HIDING OF THE SURFACE WHEN PAINTING
INTERIOR WALLS, DOORS AND TRIM

A. Be Sure All Surfaces Are Properly Prepared So That Paints Will Apply in a Heavy Coat and Will Not Tend to “Crawl”

1. Remove all dirt, hand prints, etc.; rinse off all residual cleaning agent.

2. Sand any glossy surfaces using very fine grit garnet paper. Follow safety guidelines if lead paint is suspected. Remove residual sanding dust.

B. Apply an Appropriate Primer

1. For latex paints, use PVA latex drywall primer for walls, or a latex stain-blocking primer for walls, trim and doors; alkyd primers for oil-based paints.

2. Apply primer at a sufficiently low spread rate, as recommended by manufac-turer; generally will not exceed 450 sq. ft./gallon.

3. Tinting the primer toward the color of the paint will be helpful. Do not exceed primer manufacturer’s recommended level of tinting colorant. For dark, red- or brown-based paint colors (e.g., cranberry and burgundy), a gray primer will enhance hiding.

4. If changing colors, particularly from dark to light, and especially with brighter, organic colors, two coats of paint are recommended if a primer is not used, and may be necessary even if a primer is applied, depending on the nature of the paint.

C. Consider the Nature of the Paint and Its Color:

1. Broadly speaking, all else being equal, flat paints will tend to hide better than will paints of higher sheen or gloss.

2. Top quality paints will hide better than economy products because they tend to go on thicker, shrink less upon drying, flow out better without thin spots in brush marks, and have higher-hiding pigments.

3. Very deep colors made with bright pigments may not hide well unless a suit-able primer is applied.

D. Utilize Quality Application Tools:

1. Quality brushes and roller covers put paint down in a heavier, smoother coat that hides better, than do economy applicators.

2. When brushing latex paints, apply a heavy coat with only 2 or 3 strokes (do not keep working the area), refill the brush, and move on.

3. When rolling latex paints, keep the roller full, use vertical strokes, and do not overspread the paint.