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March 11 - March 17, 2013

What is the best way to repair localized coating breakdown on steel?



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Selected Answers

From Stephen Bothello of Jotun Paints on March 15, 2013:

     Any localised coating breakdown or area where the integrity of coating is compromised requires a thorough survey by the coating manufacturer's technical representative. The coating manufacturer is the main responsible party to offer the best solution under the conditions the coating is exposed to.

     In general, determining the best way to repair coating breakdown on steel depends on the coating/ coating system used, the service environment/ operating conditions, safe access (eg. confined space), coating characteristics (with respect to overcoating, drying and curing), level of cleaning and roughnesss required, and lastly even aesthetics (for exposed steel, e.g., airports).

     Many times the repair coating/ coating system may differ, as the original coating system may not be suitable for repair under the service conditions or due to the intrinsic coating properties. An example is using epoxy zinc rich primer to repair a zinc silicate primer.

     Many coating manufacturers recommend surface-tolerant coatings that can adhere strongly to mechanically cleaned areas (SSPC-SP 3, SSPC-SP 11) with their superior wetting, penetration & adhesion properties, finishing up with the original topcoat (to level and match with the original coating). In some cases, a full topcoat over a specific area may be required to reduce the patchy spot repair appearance (for aesthetics). Several coating systems, like glass-flake-reinforced polymeric (epoxy/vinyl ester/polyester) systems, will not adhere to themselves or the steel without exhaustive cleaning procedures (like spot abrasive blasting), ensuring surface roughness for adhesion. For most coating systems, if not all, the interface between the original coating system and the repair coating needs to be feathered (accomplish a gentle sloping edge) for overlapping without risk of lifting the original coating. A feathered edge can be checked by gently scraping with a blunt-edged scraper to ensure there is no further loose coating.

     Many times, building high dry film thickness in individucal coats is a challenge when using brush/ roller techniques. In such cases, a filler may be proposed by the manufacturer (e.g., intumescent mastic filler for intumescent coating systems). In summary, due to the gamut of coatings/ coating systems available, the multitude possibilities of the environments in which they are used, it's always best or wise to check with the manufacturer. One repair method may not be suitable or  may not be the best possible solution for all, as each case needs to be seen in individual perspective.

From Brian Chapman of Cadillac Fabrication on March 15, 2013:
     I find most of these forum questions impossible to answer any way but with more questions. Rarely is enough information provided. MY first question in this case would be "what is causing the breakdown"?

From Kevin Leigh of Certified Coating Specialists on March 12, 2013:
     My recommendation? The intent of my answer to the obviously vague question was to recommend asking questions. The possible answers to the question of "what is the best way to repair localized breakdown on steel" are as many and varied as can be conceived. Initially one might ask "who" is asking the question here? The owner, inspector, engineer, contractor? Each entity would have different drivers and requirements. The same question can then be answered differently by each of the same entities. Assuming the manufacturer only "makes paint" and might not have any input, knowledge or testing on how best to repair their coating is closing a door to a potentially valuable resource. I cannot comment on your very specific situation as again, there are specific requirements that led to your specific conclusion. Perhaps the use of the particular coating was improper from the get-go, as the concept of future repair was either overlooked or perhaps the cost of spot abrasive blasting over power tooling was prohibitive. This, I feel, only makes the point of "ask questions." Respectfully,

From ron lewis of corrosion management on March 12, 2013:
     Kevin, your first paragraph is a QUESTION about the question. In your second paragraph you have passed the question back to the coating manufacturer. So, first point, why the inspector. The coating manufacturer has no idea how to repair the localized failure--they make paint. Using your recommendation insures continued failure of the paint system. First, we need to know the type of coating used orignially and the type being used for the repair. Then we need to know the environment in which the repairs are being made.
     We have seen repairs of glass-flake coatings made in a sea environment. Solvents don't soften the glass-flake coating! Power tool cleaning polished the metal substrate. The re-applied glass-flake coating had additional solvents added to soften the surrounding coating and mixed with the sea air, causing blistering. The result was, "It would have been better to leave the damaged area damaged rather than increasing the size of the damaged area and  starting more under-film corrosion.

From Kevin Leigh of Certified Coating Specialists on March 11, 2013:
     One could also ask, "How long is a piece of string"? What does the specification say for local repair? Does the manufacturer's data sheet or technical sheet have any input? What are the expectations for durability, performance? What environment is the repair to operate in? How long must it last? How much does the owner have to spend? I know this is answering a question with a question.

From Tom Selby of Rodda Paint Corporation on March 11, 2013:
Following SSPC-SP 1, Solvent Cleaning, and then SSPC-SP11, Power Tool Cleaning to Bare Metal,  would be the best way to make a localized coating repair. Then reapply the coating system to the repair area based on the coatings manufacturer's recommendation.

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Tagged categories: Coating failure; Steel


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