Ask the Expert Question-and-Answer Archive by Ron Joseph April, 2008 Coating Density Q: I am filling out an Air pollution control form for our county. On the form it is asking me for the coating density (lbs/gal). How do I know what the coating density is? The reports we print monthly give me the monthly VOC content used, however, there is no place that shows the density. Hope you can help! A: The coating density is nothing more than the weight of one gallon of the paint. Every MSDS provides this information, but it doesn't always refer to it as "density". Often, it simply gives you WPG in lbs/gal. WPG = "weight per gallon", which is the same as its density. Some MSDS provide the S. G. (specific gravity) of the coating. S. G. is the ratio between the density of the coating and the density of water. In other words, how much lighter or heavier is one gallon of the coating than one gallon water? We know that the density of water at room temperature is 8.33 lbs/gal (some reference books give it as 8.34 lbs/gal). Therefore, if the S. G. is 1.4 (no units), it is 1.4 times heaver than water, and the density must be 1.4 * 8.33 lbs/gal = 11.66 lbs/gal. Similarly, if the S. G. is 0.97, the coating is 0.7 as heavy as water (it is lighter) and its density is 0.7 * 8.33 lbs/gal = 5.83 lbs/gal. Referring to the MSDS of all your paints, you should now be able to locate or calculate the density. Good luck, Ron Joseph

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