Mil Thickness, Estimating, Etc.

There was an article recently in PaintPRO magazine on the relationship between solids content of materials and dry film thickness. The article was well done and emphasized two very important points: that the solids figure we deal with is the solids by volume (SBV), not solids by weight; and that film thickness should be measured by dry film thickness (DFT), not wet film thickness.

The article, however, did not go far enough in explaining the importance of SBV and DFT (otherwise known as mil thickness) to the painting contractor. Generally the formula to convert these two figures into “spread rate” is not of too much interest when dealing with conventional commercial products such as latex or alkyd paints. You can usually feel safe using the spread rate given on the data sheets.

But when dealing with high performance architectural or industrial coatings, with very specific DFT requirements at a high coats per gallon, compared to conventional products, it is in the contractor’s best interest to make a comparison or evaluation of the products under consideration.

Let’s see how this would play out if we are comparing two different products from a cost standpoint. The formula for determining spread rate for a product at a specific mil thickness is:

l604 x %SBV = spread rate
DFT

The 1604 figure represents the square footage that one gallon of a liquid will cover at one mil dry film thickness. This figure is constant in the formula.

For our evaluation of the two products, we will use the following data:

• The mil requirement is 2.5 DFT.
• Brand A cost $43.50 per gallon, 46% SBV
• Brand B cost $44.75 per gallon, 56% SBV

OR

Based on 20,000 sq/ft of surface to be coated:

Brand A 20,000 @ 294 SqFt/Gal = 68 Gals @ $43.50 = $2,958

Brand B 20,000 @ 358 SqFt/Gal = 56 Gals @ $44.75 = $2,506

There is, as you can see, a substantial difference in the final cost, demonstrating that the less expensive per gallon price is not necessarily the true cost of the product. Obviously this result will not be the case in every evaluation we make, but this sort of difference does occur often enough that it makes good sense to run the numbers, particularly if there is a substantial per gallon price difference.

You will note that the spread rate number we arrived at is noted as theoretical. That means that a waste factor has to be added. This might vary from 5 percent to 20 percent or more depending on the method of application: brush, roll or spray. Also, you will note that throughout this article, we have used the term “spread rate” as opposed to coverage. This implies how much surface a product will spread at a specific DFT, as opposed to coverage which implies the hiding ability of a product.

If you have a specific question or problem area you would like to have discussed, contact Len Hijuelos at P.O. Box 2585, Gretna, LA 70054 or e-mail.