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May 4 - May 10, 2015

When performing maintenance painting of tank coatings, what are the pros and cons of overcoating an existing lining that appears to be in good condition?

Selected Answers

From Warren Brand of Chicago Corrosion Group on May 14, 2015:
Joe, I like your thoughts very much. However, I'm surprised at your 25% cost figure. Are you saying that if you're dealing with, say, an existing 40-mil tank lining,  the cost between brush blasting the existing material and applying, say, a 10-mil topcoat, is going to save the owner only 25% over completely removing an existing 40-mil coating, providing an SSPC-SP 10 with a 2.5 mil profile and reapplying a 40-mil topocat? I would think the cost different would be closer to double.

From joe friedt of advanced painting& Sandblasting Inc. on May 5, 2015:
You used the words “tank lining,” which to most of us means tank interior coating, and you are describing the interior over- coating of an existing interior tank coating. But everything you described is perfect for exterior overcoating/repainting.

For the interiors of steel tanks, I run into this dilemma weekly. I'm totally honest with the customer in explaining the difference in overcoating and total removal. I tell them overcoating should work in most cases, but my paint supplier will not stand behind an overcoat interior. And yes, it may be 25% less cost to prepare and coat over the existing coating, and I will give you a 1 year warranty.(with sleepless nights). But a total removal with that 25% cost added back in, I may give you up to a 15- year warranty, and you will have the written blessing of the coating manufacturer.

With the amount of fly-by-nights and unscrupulous contractors that change names after every failure, owners should want the paint manufacturer on their side. When considering the potential problems of delamination, contaminated cargo, another shutdown, filter clogging, and possibility of structural damage to the steel tank if peeling to bare metal occurs due to undetected fracturing. The owner should always chooses total removal.

From Warren Brand of Chicago Corrosion Group on May 4, 2015:
We have designed systems to overcoat perhaps millions of square feet of existing coatings. And overcoating gets a bad rap for a lot of reasons. First, there's more money in complete removal and replacement. Paint manufacturers typically sell less paint; contractors don't make as much money because of less surface prep and application costs as overcoating systems are often only one or two coats. Architects and engineering firms, conservative by nature and often earning fees based off of the total cost of the job, may not have the depth of technical understanding required to test an existing substrate, or may simply not be incentivized to do so. Paint and coatings don't know what they're adhering to. Paint and coatings adhere to substrates via a mechanical bond (though some argue that there may be other bonding properties as well). Technically, I can see no downside to overcoating an existing system, as long as it is a good candidate for overcoating. The pros are that it saves owners time and money. One of the biggest technical mistakes I've seen is that people test an existing coating system before surface prep. A critical step is to test the existing coating system AFTER surface prep has been carried out. This ensures that the surface prep has not fractured or otherwise damaged or interfered with the adhesion of the existing coating system to the substrate. Then, of course, pull tests must be carried out (we do them before and after surface prep) to quantify adhesion, then design an overcoating system which will not impart undue surface tension on the existing substrate. One can even perform a proof of concept to ensure all technical details.

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Tagged categories: Asia Pacific; EMEA (Europe, Middle East and Africa); Latin America; Linings; Maintenance coating work; North America; Quality Control; Tanks

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