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June 5 - June 11, 2019

I have several water tanks constructed with carbon steel for demineralized water storage. The tanks are about 20 years old, and the original coating has started to blister. For refurbishment of the coating, what material should be used and at what dry film thickness? The original type of coating is unknown.

Selected Answers

From Warren Brand of Chicago Corrosion Group on June 13, 2019:
Hi Larry, I agree 100%. Quick comment about an epoxy filler for pitted areas. We never use fillers. They are time-consuming and costly. We typically spray (when I was a contractor) into the pits from multiple angles (much like one would with stripe coating) during the spray process. And then spray the entire tank (1-coat system) at between 40 to 60 mils and, in some cases, 50 to 80 mils. And, of course, spark test. Did we get runs? You bet. They were cosmetic and in no way affected the performance of the coating system. Back in the day, our firm provided 10-year non-prorated guarantees. And almost never had a problem -and if we didn't, it was not related to runs. Here is a link to an online feature for PaintSquare with photos of a tank I lined decades ago with runs and pits.

From Larry Muzia of Exceletech Coating & Applications, LLC on June 12, 2019:
We recently completed a demin water storage tank and found it contaminated with chlorides....go figure. After removal of the failed coating and insuring chlorides were also removed, we applied a primer followed by a 30-mil finish coat of a highly cross-linked epoxy, followed by high voltage holiday testing. Tank was severely pitted as well, which required an epoxy filler. In all immersion type linings, following protocol from start to finish is a must to achieve a long service life. As a contractor and inspector, I see way too many times poor workmanship that gets overlooked by the ill-informed.

From Aajjay Sunkay of ASSETREIFURB ENGINEERRS on June 10, 2019:
If you are looking for an expected life of the coating to be more than 15 years,  following are my recommendations.  Internal of tank: Remove the old coating by abrasive blasting. Prepare the surface to SA 2.5 with surface profile of between 50  to 75 microns. Then apply glass-flake-filled novolac epoxy vinyl ester-based coating up to a DFT of 800 - 1000 microns in two coats. External coating: Remove all loose rust, oil, grease, dust and any contaminant by manual preparation method. Apply inhibitive, rust-converting, zinc-rich, polyamide-cure glass flake epoxy as.primer at a DFT of 150 microns, followed by two coats.of polyamine-cure epoxy coating at a  DFT of 275 microns

From Warren Brand of Chicago Corrosion Group on June 10, 2019:
This is an incredibly complex question. And I agree with Zenith generally, but not specifically. For the last 40 years our speciality has been lining water tanks. And to say to apply a solvent-free epoxy is like telling someone to go buy a car with four wheels. Not all 100% solids (solvent-free) coatings are the same - not by a long shot, particularly when talking about demineralized water. If you took 20 different solvent-free coatings and applied them in exactly the same manner, you would have a bell-curve of performance. And it takes an enormous amount of expertise to identify an "optimal" system. By the way, we would recommend a one-coat system.

From Zenith Czora of Master Builders Solutions on June 6, 2019:
For complete refurbishment, total removal of existing coating is required; fabrication defects need to be removed and replaced; and adequate surface preparation must be completed. Incorrect surface preparation will lead to coating failure. For tank linings, 2 coats of medium build, solvent-free,  2- pack epoxy is recommended. Each coat must be applied at 250 - 300 um dry film thickness,  resulting in a coating system of 500 -600 um dft. For coating the outside surfaces of the tank, the following coating system is recommended: 75 um dft of 2-pack epoxy zinc primer, 200 um dft of medium-build, 2-pack epoxy, 50 um dft of 2-pack polyurethane topcoat.

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