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I’m not a fan of spot abrasive blasting for repairs on existing structures. Don’t get me wrong, I think that abrasive blasting is the best surface prep you can perform, but only if it’s on a complete or new structure.
Once you have an aged, in-place paint job, spot repairs of spot failures require a different technique. Otherwise, you risk turning a one-square-inch failure into a circular one-square-foot failure (or more).
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Funtay / Getty Images |
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Any spot-repair method needs to remove all active rust and rust scale, to remove all surface contaminants (dust, dirt, oil and grease) and to remove any compromised or dis-bonded existing coatings, ideally feathering to remove sharp edges of thick coatings. All these criteria can be met with mechanical tools. And if a profile is required, specialized tools can provide that, too.
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Many reasonable experts might choose to disagree—and I readily assent that this is not an absolute or binary issue—but I do think the topic is worth a robust discussion, so I’ll start with my premise.
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First, it’s really tough to properly contain the dust and debris from a blasting operation in an operating plant environment. Even with the best of intentions and efforts, fine dust will work its way into seals and rotating equipment with negative consequences to their operation and lifetimes. These unintended consequences can be invisible and far-reaching.
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Second, abrasive blasting often causes significant damage to existing sound coatings surrounding the failure, often far from the site of the repair. Abrasive blasting is not a precision instrument. Starting and stopping a blast nozzle with many feet or meters of hose takes time. Abrasive streams from the nozzle during that shutdown period can cause overblast in the intended repair area or inadvertent spray of high-velocity abrasive far from the intended repair.
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Finally, abrasive blasting is not an effective or ideal tool for feathering existing sound coating. It can produce what looks like a feathered edge, but the outside perimeter—where the feathered versus un-feathered coating interfaces—is subject to random impacts from abrasive particles that tend to fracture weathered and brittle existing coatings. Too many times I have seen the evidence of prior spot repairs from the circular rings of failure at the outside edge of the blasted areas.
In my experience and judgement, mechanical cleaning and mechanical impact tools, such as sanders, grinders, needlescalers and Bristle Blasters are better tools for repairs.
It also makes sense to consider alternate coatings, like epoxy mastics, for industrial repairs. If one has a structure that was painted 15 years ago with an intended coating life of 25 years, you probably just need the spot repairs to last for 10 more years, until the entire structure is repainted. Spot repairs preserve the structural integrity of the steel, but don’t need to last as long as the original design life.
Any spot-repair method needs to remove all active rust and rust scale, to remove all surface contaminants (dust, dirt, oil and grease) and to remove any compromised or dis-bonded existing coatings, ideally feathering to remove sharp edges of thick coatings. All these criteria can be met with mechanical tools. And if a profile is required, specialized tools can provide that, too.
But in most cases, abrasive blasting isn’t the best, or the most cost-effective, surface preparation for spot repairs.
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| ABOUT THE THE BLOGGER |
Robert Ikenberry |
Robert Ikenberry, PCS, has been in industrial painting and construction since 1975. Now semi-retired as the Safety Director and Project Manager for California Engineering Contractors, Robert stays busy rehabbing, retrofitting and painting bridges. His documentary on the 1927 Carquinez Bridge was the pilot for National Geographic’s Break it Down and an episode of MegaStructures. |
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SEE ALL CONTENT FROM THIS CONTRIBUTOR
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Tagged categories:
Bridges;
Program/Project Management;
Abrasive blasting;
Abrasives;
Asia Pacific;
Blasting;
EMEA (Europe, Middle East and Africa);
Latin America;
North America;
Spot repair;
Surface Preparation;
Surface preparation
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Comment from Ted Valoria, (8/7/2019, 11:48 AM)
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Robert,
I appreciate your comments and observations about your experiences with abrasive blasting for spot repairs. However, I would suggest that you consider using sponge media blasting to effectively repair damaged coatings without damage to existing, adjacent, coatings.
Sponge-Jet surface preparation technology has been successfully used for many years to accomplish precisely what you are discussing in your blog. In fact, one US Navy Shipyard even referred to sponge blasting as a "surgical blast process" to repair coatings.
In regards to your first point, sponge media blasting eliminates up to 95% of the emissions normally created by dry abrasive blasting, so the dust can be easily contained, and environmental issues abated. Sponge blasting is an ideal process to use around rotating equipment, valves and electrical boxes because of the low dust and the unique feature of no ricochet of the abrasive, which normally causes entrainment of the abrasives in valves, mechanical seals and electrical motors resulting in premature failures.
To your second point, sponge media does not ricochet and therefore does not damage surrounding coatings like ordinary abrasives. As long as the operator keeps the nozzle pointed at the area being repaired there is no damage to adjacent coatings during a start and stop process. If an electric deadman were used rather than a pneumatic deadman then the blasting process would stop almost instantaneously. These methods would eliminate overblast.
In point three you suggest that in your observations the abrasives damage feathered edges, and that may be true, but with sponge media, the abrasives are encapsulated in a foam material which allows the operator to perfectly feather the edges without shattering the intact coating.
I would also like to add that it has always been the opinion of this organization (SSPC), and others, that the best method of surface preparation for coating adhesion and surface cleanliness has been to abrasive blast with an angular abrasive, to achieve the best surface for a coating application.
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Comment from Damon Givens, (8/7/2019, 1:04 PM)
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I think this is pretty reasonable given the setup required for performing and containing abrasive blasting. I work in a food facility where the dust and debris from blasting is just not acceptable. Additionally, our facility painting includes a lot of spot repairs scattered all about the plant. A crew with the right hand and power tools can move around rather rapidly in this environment and still do high quality work when blasting is not an option.
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Comment from Garth Moran, (8/21/2019, 5:54 PM)
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A very subjective topic. It really is horses for courses e.g. if a big tank has to be scaffolded up ( = large cost) why wouldn't you spend the time and effort to do spot blasting. Also dependant upon environment that you are in, arid/mild environments you will get decent performance of power tool prep (SP3/11/15) out of most surface tolerant coatings, however in coastal zones or offshore, these would typically only last less than 5 years if you are lucky. Just a question of the right methodology for the right situation to give the desired results.
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Comment from Sukhmandir Singh, (8/24/2019, 3:12 AM)
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Bristle-Blaster could be an ideal Powered tool for Spot repairs.
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Comment from Michael Beitzel, (8/26/2019, 10:03 AM)
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Robert I am with you on this especially when it comes to preparing mechanically galvanized bolts in the field on shop primed or fully painted steel
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Comment from Wayne Tupuola, (11/15/2019, 10:05 AM)
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Laser cleaning systems provide the lowest operating cost of all industrial de-painting or surface treatment on spot repairs or surface prep areas of less than 144 cubic ft at a time. Lasers are many times faster than the use of chemicals or abrasives, lowering setup time and labor costs. Large material and disposal costs are eliminated because the laser leaves no waste or debris. Often returns on the investment can be seen in less than a year.
Laser surface cleaning systems work by aiming a powerful high-pulsed laser at the rusty surface. This energy disintegrates existing contaminates and effectively removes them from the substrate. It doesn’t continue to burn away the solid surface underneath because metals reflect light very well especially in the infrared spectrum. What’s left is a clean and paintable surface.
Laser, unlike sandblasting, is selective, making it easy to focus on a very small spot or strip with no effect on adjacent materials. Made in America, the industry leading Laser Photonics’ CleanTech product line is the answer to safely and efficiently removing rust with impressive results.
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Comment from Gisle Solhaug, (2/4/2020, 5:28 PM)
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One option is Paint removal by Electromagnetic Induction.
Our electromagnetic induction machine removes coatings up to 1-inch-thick at a rate of 200 -400 square feet per hour.
RPR electromagnetic induction disbonding does not generate airborne particles or noise. RPR provides continuous operation, no disruption of other inspection or maintenance work, and safe containment of hazardous coatings like asbestos, lead, and PCB. Reduced risk of contamination to air, water, or soil and considerably less energy consumption benefits both the local and global environment. Contact me for details, gs@rprtech.com
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Comment from Steven Ambriz, (5/11/2020, 8:34 AM)
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Depending on the area that requires spot blasting Schmidt® Blast and Recovery Systems can be effectively used to and will mitigate the issues you have mentioned. 1) The BRS systems will contain the dust and debris. 2) Because of the nozzle proximity to the surface the blasted area can be easily controlled. 3) The BRS Systems integrated pressure regulator will provide the means to feather the edges. You also have the option to change the abrasive to better control the feathering. As with any blast operation the shorter the blast hose the more efficient it will be. In the cases where a long blast hose is necessary all Schmidt equipment can be provided with an abrasive cutoff feature that allows purging the blast hose of abrasive prior to ending the blast operation. The Schmidt BRS Systems are available in various sizes from 0.46 cubic foot capacity to 6.5 cubic foot.
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Comment from Nguyen Tung, (10/1/2020, 7:54 PM)
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Very helpful article, and thanks great comment from Wayne Tupuola.
Thanh tung from cat tuong jsc
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Comment from Dave Wonnacott, (11/18/2021, 4:05 PM)
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Excellent article and some very good response comments. However the one opportunity I did not see was the use of gelled acids for precision of application and measured etching and removal.
It would seem like there is more than one approach based upon the specific job.
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Comment from Traian Rus, (2/7/2022, 2:31 AM)
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Hi, I propose to keep in mind dry docking: spot blasting is common and many times there is no time to stop blasting while moving from one spot to another thus may ricochets and/or intact areas are affected. From practical point of view, this is it. The alternative, in my opinion, may be hydro-blasting providing that there is sufficient profile. Otherwise, we cannot avoid spot blasting: it would be too slow. Power tooling (for big areas) it would be too slow and profile will be low.
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Comment from Lydia Frenzel, (2/9/2022, 2:16 PM)
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I was surporsed to see a comment after this length of time from the original publicaion. Very good discussion. At advisorycouncil.org - to go downloads (upper left) go to item 12- a detailed FHWA discussion on many different types of "spot" or partial removal based on bridges- but the general concepts are the same. It can be used to think about complex structures, a little area deteriorated to a lot of area deteriorated. One report is a working excel sheet for economics.
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