November 21 - December 4, 2011
How do you measure the DFT or WFT of a film-forming sealer applied to a metallized coating (e.g., flame-sprayed zinc)?
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Simon Hope of Bilfinger Salamis on
November 30, 2011:
The answer is in the question! For TSA (Thermal Sprayed Aluminum), we are looking to seal an inherently porous material. The sealer coat is there to fill the interstices but not to create any particular build. The amount of sealer applied is dependent on the level of porosity and thickness of the TSA as well as the solvent content of the sealer coat.
Guidelines are given by manufacturer, but these are only ballpark figures to form the start point for application. Normally, a sealer is applied just enough to create an apparent wet film, which will dry back into the profile as the solvent evaporates, thus coating the bare metal as well as providing a tie coat for subsequent higher volume solids coats. Consumption can, if required, be calculated in area per unit volume, and from that, it is possible to approximate to WFT and DFT, though, in reality for this process, these are pretty meaningless and the visual appearance is more relevant. Coating integrity is the primary objective, and a good applicator will produce the visually correct sealed substrate, which can be confirmed by the material consumption.
Gunnar Ackx of SCICON worldwide bvba on
November 22, 2011:
You can't. A sealer for spray-metallizing is typically applied on visual observation. The idea is to just cover the spray-metallizing without actually building up an (entirely) closed film of paint over the spray-metallizing. Ideally, you will still see the surface roughness of the spray-metallizing, but not its color anymore. Also key to a good sealer-coat application is using the correct amount of thinning, so that the sealer-coat gets soaked into the pores of the spray-metallizing as well as possible. The amount of thinning will vary from one product to another and from manufacturer to manufacturer, and I've seen this vary between 5 and 50%. If you're not using enough thinner, then the sealer-coat will lay on top of the spray-metallizing, which will cause it to have much less adhesion and will likely still give you the typical 'popping phenomenon' (outgassing of the spray-metallizing), which is one of the main reasons why a sealer-coat is applied onto spray-metallizing.
David Lemke of Team Industries, Inc. on
November 21, 2011:
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I have used the theoretical coverage of the coating, which included an estimated transfer efficiency with the amount used. I gave the applicator a low and high amount of coating he could use for the square footage they were coating for the range of the DFT. I took the actual amount they used and plugged it into the formula to get the estimated DFT. Another way is to put a piece of substrate across the area to be coated and have the applicator spray the amount they thought would be correct across both the metallized coating and the piece of substrate. Wet mil the piece of substrate, and that should tell you if the thickness is on track to meet the DFT. If not, then you can adjust. The thing with both ways, you need to have an applicator who can consistently duplicate the application process to be effective.
Coating / Film thickness;
Dry Film Thickness (DFT);
Wet film thickness
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