How much does that can of paint really cost?

When comparing paint types, it is important to consider factors beyond the cost per gallon. This article, and its handy chart, will help you determine the best paint you can get for your money.

By William Hansen and Michael Brown

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When you try to compare the cost of one paint to another, consider factors other than the cost per gal. Additional factors include coverage rates, material cost, cost per square foot and paint film performance. What you need is the true total cost of painting -- the cost per square foot per year of protection. This life-cycle cost is the total cost of the painting project divided by the useful life of the coating system. Conventional wisdom says that materials represent 20% to 30% of the painting cost with 70% to 80% going into surface preparation, application and various miscellaneous costs. As you can see, the coating’s unit cost is a substantial part of that equation.

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Do the math
Start with the coverage rate and material cost per square foot Remember that one gallon is equivalent to 231 cubic inches. If a solid of this volume was sliced into sheets having a thickness of 1 mil (0.001 inch) and laid side by side, it would cover an area of 1,604 square feet But, paints aren’t isotropic solids. Most contain solvent or other volatile organic compound that changes the math. These materials evaporate and leave no film-forming pigment or resin. Applying a 1 mil wet film thickness of a paint having 50% solids by volume will produce a dry film thickness (DFT) of only 0.5 mil. At 30% solids, the DFT would be 0.3 mil.

The relevant variable is the coverage rate -- the product of 1,604 and the percent solids by volume -- which has mil-square feet/gallon as the unit of measure. For example:

• 1,604 square feet/gallon x 50% volume solids = 802 mil-square feet/gallon.
• 1,604 square feet/gallon x 30% volume solids = 481 mil-square feet/gallon.

To determine the area you can cover with one gallon, divide the coverage rate by the desired DFT. For example, the area you can cover with a DFT of 5 mils using a paint with 50% solids is (1,604 x 50%)/5 = 160 square feet per gallon.

Because you’re not painting a sheet of perfectly smooth glass, other factors affect this theoretical coverage rate. Surface configuration, blast profile, overspray and product waste reduce things further. Application methods can produce losses ranging from 5% to more than 50%.

For example, brush and roller application can result in 5% to 15% material loss, while spray applications can waste from 15% to more than 50%, depending on the orientation of the substrate relative to the air flow direction.

Higher math
Find the material cost per square feet by dividing the cost of a gallon of paint by the coverage rate. This might seem insignificant, but consider two alternate polyamide epoxy paints. Paint A, at a cost of $25 per gallon, has 75% volume solids. Paint B, at $20 per gallon, has 45% volume solids.

Paint A offers 1,604 x 75% volume solids = 1,203 square feet per gallon coverage at 1 mil DFT. The unit cost of $25 per gallon divided by 1,203 = $0.021 per square foot material cost at 1 mil DFT. But the recommended dry film thickness is 3 mils, so multiplying $0.021 by 3 yields the true cost of $0.062 per square foot for Paint A.

Paint B has 45% volume solids, so it offers 1,604 x 45% volume solids = 722 square feet per gallon coverage at 1 mil DFT. The cost of $20 per gallon divided by 722 = $0.028 per square foot material cost at 1 mil DFT. But the recommended thickness is 4 mils, so multiplying $0.028 by 4 gives the true cost of $0.111 per square foot for Paint B.

Even though either paint would be adequate, their cost per square foot per mil was appreciably different. On a cost per square foot per mil of DFT basis, Paint B, with the lower cost per gallon, is actually 180% more expensive than Paint A.

Getting and keeping the savings
It’s now time to put this knowledge to work so that you can capture the potential savings. Following are steps you should consider to help make your painting as cost-efficient as possible.

Calculate cost per mil

Volume solids

Price per gallon

20%

30%

40%

50%

60%

70%

80%

90%

100%

$8

2.6 cents

1.6 cents

1.2 cents

1.0 cents

0.8 cents

0.7 cents

0.6 cents

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