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February 28 - March 6, 2011

How do you measure wet film thickness on shop-applied organic zinc-rich primers to accurately predict dry film thickness?

Our shop applies a great deal of OZ primer, and we have found that WFT readings with a standard gauge are not accurate predictors of DFT.



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Selected Answers

From Larry Muzia of Exceletech LLC on April 11, 2011:
      The difficulty in obtaining accurate wet film readings with zinc- filled coatings is the metallic zinc particles can pack under the outer edges of the gage and create a false bottom, so to speak. Remember, we have a metal-filled product which will not always move out from under the wet film gage edges, thereby not allowing the gage to properly seat on the steel member. Rapid drying is another issue to consider, especially with inorganic zinc. Taking several immediate wet film readings and the application by a professional applicator can help achieve a responsible procedure to achieve the necessary results. Although organic zincs are more tolerant of excessive dft than inorganic zincs, more is not always better. Also, remember to account for magnetic base plane when varifying the dft gage.

From Guru Sankar of Sathya Hitec Solutions on April 8, 2011:
     By using the conventional wet film thickness gauge, we can arrive at two caliberation graphs. In the first one, we can maintain the same blast profile and by varying the coverage of the organic zinc-rich coating, we can get the first graph. In the second one, we can maintain the same coverage  but vary the blast profile to get second graph. By experience in correlating these two graphs, we can predict the dry film thickness from the wet film thickness measurements.

From James Prevatt of Asset Preservation Partners on March 2, 2011:
     Zinc-rich coatings, both organic and inorganic, have larger particle sizes and will not shrink like conventional coatings. The amount of shrinkage will vary with the change of particle size of zinc. Apply a gallon to the prescribed sq. ft. (i.e., 200 sq. ft. per gallon) area, carefully noting the wet film thickness. Once dry, measure the dry film thickness and you can estimate the true shrink of the coating from solvent evaporation. Should be around 60-80 percent of a normal shrinkage. Best to know in shop situation to be sure on your estimate of usage. Jim Prevatt, Lakeland, Fl.

From Tom Swan of M-TEST on March 1, 2011:
     The best way to measure the thickness of a zinc-rich coating before it drys completely is to put a shim of known thickness on the zinc coating and use a DFT Gauge to measure the thickness of the coating. Subtract  the thickness of the shim, and you should have a pretty good estimate of the thickness of the coating. It's not perfect but probably the best you will do. I haven't tried it with organic zinc coatings, but it works reasonably well with inorganic zincs.

From Antonio Tolotto of Boat S.P.A. on March 1, 2011:
     Zinc silicate shop primers are inorganic rust preventive coatings used mainly in the shipbuilding industry. These products must have excellent welding and cutting properties and are required to present no health hazard to the welder. They must be in accordance with IMO Resolution MSC.215(82) and have to be approved by the IACS Classification Societies that fix the acceptable thickness at which they can be applied without interfere on weldings performances. According to the technical data sheets of the main producers, zinc silicate shop primers are currently applied at 15 microns d.f.t., with min. 10 and max 25-30 microns d.f.t. The roughness of shot blasted surface is recommended from 30 to 75 microns (Rz). The recommended application method is airless spray in automatic plants. It is impossible to check the right thickness (wet or dry) of the shop primer with traditional methods. The roughness of the plate plays a dominant role, because surface profile will affect the coating thickness value displayed by the testing gauge. Roughness will vary from part to part and cause the instrument to overstate the true thickness of the coating. Additionally, shop primers are typically coatings that dry quickly, following the surface profile, because they must protect also the peaks of shot-blasted surface (15 microns d.f.t. must protect 75 microns!). By the way, there is a method to check exactly the exact thickness of the paint applied. This method uses a perfectly flat plate (10x50 centimeters) of steel fixed to shot-blasted surface just before the plate enters  the painting plant. After the plate dries in few minutes, collect the plate and check. The right thickness is the average of 10 measures on the 50 cm length. You obtain a number (for ex.: 28). This is NOT the real thickness of the paint applied on the shotblasted plate. This numer must be decreased of 30%: 28 -6= 22 microns. 22 microns is the right thickness applied on peaks and valleys of the shot-blasted plate. The reason is that when a flat surface is sand- or shot-blasted, the surface increases conventionally by about 30%.

From Lee Edelman of CW Technical Service on February 28, 2011:
     Organic zinc-rich primers are designed for ferrous metals and are formulated to dry fast. The spread rate is based upon volume of solids.  Most organic zinc-rich primers product data sheets recommend wet film thickness be checked, but the accuracy of wft testing of these materials is questionable. Care should be taken not to apply heavy film build during the application process. Take several wft measurements to get an overall average. Organic zinc-rich primers  are porous.

From richard d souza of stoncor middle east llc on February 28, 2011:
     Shop-applied organic zinc primers dry too quickly to take meaningful wft measurements; and, therefore, it is always advisable to prepare different sample plates with varying number of passes and take dft readings on the plates once the coating is fully cured or on overnight curing. This way,  the operator knows how many passes he should spray in order to achieve the desired dft. Obviously, the thinning percentage and tip size should remain the same, and best practice is to carry out such test panels with different sprayers and spray stations to keep variable to a minimum.

From noe garcia of siemens industry inc on January 25, 2012:
     Due to the difficulties of measuring WFT, we measure DFT using a BYKO CUT Gauge. This may not answer the question, but it is another way of evaluating if the right amount of primer is there.

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Tagged categories: Coating / Film thickness; Dry Film Thickness (DFT); Primers; Wet film thickness; Zinc-rich (organic)


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