Regardless of the substrate, some professional painters don’t have an appreciation for how important the proper spread rate is when applying paints.

That’s because it’s often possible to apply a paint at a spread rate higher than that recommended, and still achieve a uniform appearance, especially if a primer of a similar color has been applied.

For example, a painter might feel good about getting 500 square feet per gallon on a job, while achieving a good appearance. However, if 400 square feet per gallon is the manufacturer’s spread rate for the paint, then spreading it at 500 square feet per gallon will result in a dry paint film that’s only 80% as thick as is recommended.

Regardless of how good it looks, a dry paint film that is too thin can have an adverse effect on a number of key paint properties, all of which affect the quality of the finished job.

QUALITY IS COMPROMISED
Take mildew resistance as an example. Top quality latex paints provide better mildew resistance than economy latex paints do, because they generally contain more mildewcide, an ingredient designed to fight mildew growth.

If you overspread the paint, however, you minimize this benefit because a thinner coat contains less of the growth-inhibiting additive. A thicker paint film not only provides the mildew protection the paint is designed to deliver, but also helps keep the fungus from reaching nutrients in the substrate.

The detrimental effects on paint properties caused by overspreading can reveal themselves in a number of other ways, including:

• Compromised Crack Resistance — The ability of a paint to resist cracking is directly related to its dry film thickness. This is especially important when painting or re-painting exterior wood.

• Poor Hiding — The paint may not completely hide the surface, so the previous color or pattern shows through. This may become evident as soon as some of the paint film has eroded from either weathering or cleaning.

• Poor Washability — The cleaning necessary to remove dirt or stains may remove a small portion of a paint film, especially in the case of a flat paint. For that reason, a thin paint film will stand up to less cleaning than the same paint more thickly applied.

• Poor Flow and Leveling — Brush or roller marks may be evident in the dried paint. Applying a heavier coat provides better flow and leveling.

• Ultimate Durability of an Exterior Job — A thin paint film will erode sooner than a thick paint film, all else being equal.

• Sheen Uniformity — A thin application of paint can sacrifice this important property.

Primers Affected, Too
The drawbacks of not applying a coating at its recommended spread rate are not limited to paints; primers are affected, too. That’s because overspreading a primer will also result in a dry film that is too thin, which, in turn, will reduce performance in terms of its key properties.
In the case of primers, the key properties impacted by spread rate are crack resistance, hiding, mildew resistance, sheen uniformity of the finish coat and, depending on the nature of the primer, stain blocking and corrosion resistance.

So, take the time to measure sections of your paint job to calculate the area. Then determine approximately how much paint, stain and/or primer, in gallons, will be required to coat the area. Whether you are spraying, rolling or brushing, strive to apply the proper amount of coating to the surface.

The recommended spread rate will depend on both the surface (smoothness and porosity) and the particular paint or coating. Very rough exterior surfaces such as textured stucco, for example, will typically take paint at approximately 150 to 200 square feet per gallon, while smoother surfaces like sealed cast concrete and primed smooth cedar will be more like 400 to 450 square feet per gallon.

Whether you’re tackling an interior or an exterior job, always apply paint and primer at their recommended spread rates. Doing so will result in a thicker, more-protective and longer-lasting film – all of which will make your customers want to “spread the word” about the quality of your work.

Stretching paint by covering more square footage than recommended is a practice that results in a thinner coat of paint, and sacrifices key properties. Another practice that leads to a thinner paint film is thinning or diluting the paint.

Thinning is a concern because of its effect on the application and performance properties of the paint. For example, if a latex paint is thinned with water before use and applied at the recommended spread rate, paint coverage, as measured by dry paint film thickness, will be reduced because dilution lowers the volume of solids of the paint. This can result in a reduction of the same properties described in the adjoining article on overspreading paint.

Dilution may also interfere with the balance of paint properties, particularly flow and leveling versus sag resistance. Thinning a paint can cause it to sag on a vertical surface, tempting the painter to apply less paint, which again results in a thinner dry paint film.

So, do not thin paints unless necessary, (as is sometimes required for application by spraying), and then, only according to the paint manufacturer’s instructions.

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