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Question posted - June 5 to June 11, 2016:

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

Selected Answers

From Scott Sammons of Independent contractor at large on June 15, 2016:
The word is always dangerous, given that the definition of "nominal" is often "in name only," as in the OSHA spec. for four-inch toe boards on scaffolds, but the common perception of the word is minimum.

From Patricia Engelbert of Level 3 Coating Inspection, LLC on June 10, 2016:
I have a different interpretation of what the question is asking. First thing I would do is to consult the Product Data Sheet (PDS) to determine if the spec requirements for dry film thickness are in agreement with the range stated in the PDS. If so, I would get clarification from the owner for the maximum allowable DFT so that a reasonably acceptable range can be determined. If not, I would discuss with the owner the potential issue(s)and find out their logic for this wording. If there is no response.... run... run far and fast!

From Steve Brown of EDF Energy Ltd on June 10, 2016:
I'm wondering if the questioner is really looking to know what acceptance criteria will be applied to the work, in other words, in effect how nominal dft is defined. This can only be agreed between the contracted parties, but this is usually done by reference to an appropriate standard. Here in the UK one would commonly cite BS EN ISO 12944 Pt 5 (2007) for the coating system selection, which includes the following definition .... Unless otherwise agreed, the following acceptance criteria, as stated in ISO 19840, shall apply: the arithmetic mean of all the individual dry film thicknesses shall be equal to or greater than the nominal dry film thickness (NDFT);  all individual dry film thicknesses shall be equal to or above 80 % of the NDFT;  individual dry film thicknesses between 80 % of the NDFT and the NDFT are acceptable provided that the number of these measurements is less than 20 % of the total number of individual measurements taken;  all individual dry film thicknesses shall be less than or equal to the specified maximum dry film thickness. Other standards (NACE/SSPC) I believe use similar but not identical acceptance criteria, hence why agreement is required in advance.

From Robert Audlee of Stainless Steel Coatings on June 6, 2016:
The contractor should use the "volume solids" percentage for the coating to calculate the appropriate Wet film thickness and apply the coating accordingly, using a comb-style wet film thickness gauge to check his work while applying. The contractor should also ensure that actual coverage is reasonably close to theoretical coverage minus waste, to avoid under- or over-application. In industrial coatings, film thickness is critical.

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Tagged categories: Coating / Film thickness; Contractors; Dry Film Thickness (DFT); Specification

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