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May 17 - May 23, 2010

How should a contractor respond when he reads a specification requirement that "the nominal dry film thickness of the coating shall be 3 mils"?

Selected Answers

Nominal DFT is the final achieved thickness of the coating system, and there is always range (up and down) to be considered. Nominal DFT can be controlled by proper and regular checking of WFT and the ratio of thinning during paint application.

From AJ Holst of Holston Supply on May 28, 2010:
Nominal for me as a distributor means minimum necessary to achieve performance characteristics. While the spec may give some leeway to the installation contractor, it is my responsibility to educate the contractor what the minimum DFT is for the particular product based on the application.

From Mike Hyde of Global Coatings Inc. on May 27, 2010:
I have always understood nominal to be the target dft and would expect to be in the 80% under & 20% over range. HOWEVER, I always ask my customer the intent, and most often the response is that nominal means minimum.

From chao wei of ABS on May 24, 2010:
I agree with Roy Bradley's answer. Nominal DFT is a minimum total dry film thickness of the coating system after it has dried and cured. Thickness measuremenres shall follow a certain rule, for example, a 90/10 rule (90% of all measurements shall be greater than or equal to 3 mils and none of the remaining 10% of measurements shall be below 90% of 3 mils or 2.7 mils). Measurement check points and maximum DFT allowed should also be specified in the coating specification.

From Wayne Salisbury of Conproco corp on May 18, 2010:
I would ask the coatings manufacturer what wet film thickness is required to achieve 3 mils dry, then have my applicators use a wet film gauge to certify we were applying the coating to specifications.

From John Linder of Odle, Inc. on May 18, 2010:
Nominal means, "of or relating to," or "approximate." As stated, this would be taken to mean a "target" DFT with the understanding that there is some variation in the coating application process. However, I would recommend that you refer to the coating manufacturer's application data sheet for the coating being applied. If 3 mils DFT is listed as the minimum, then you should make the 3 mils DFT as the absolute minimum. In other words, do not violate the manufacturer's parameters as listed on the data sheet. I have seen many specifications where the engineer, or specifier, wrote "nominal" when they meant minimum and the application data sheet of the coating confirmed it. Most specifications also have an all- inclusive escape clause for the specifier which states that the more stringent of the conflicting information will prevail.

From Roy Bradley of Laing O'Rourke on May 18, 2010:
Reference to "nominal" dry film thickness in paint specifications recognises the fact that a variation in film thickness is inevitable. The specification should qualify "nominal" by stating that over any square metre the average of dry film thickness measurements taken should equal or exceed the nominal thickness and no reading should be less than, say, 80% of the nominal thickness. If this is not included in the specification, then it would be prudent for the contractor to confirm and, perhaps, feel justified in suggesting, say, 70% of the nominal thickness based on an experienced assessment of the paint systems capability. Wet film thickness gauge monitoring should be an expected quality control measure.

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