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The Case Against Abrasive Blasting for Spot Repairs

TUESDAY, AUGUST 6, 2019

By Robert Ikenberry


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).

Funtay / Getty Images

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.

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.

  • 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.
  • 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.
  • 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.

ABOUT 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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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

Comment from Ted Valoria, (8/7/2019, 11:48 AM)

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.


Comment from Damon Givens, (8/7/2019, 1:04 PM)

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.


Comment from Garth Moran, (8/21/2019, 5:54 PM)

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.


Comment from Sukhmandir Singh, (8/24/2019, 3:12 AM)

Bristle-Blaster could be an ideal Powered tool for Spot repairs.


Comment from Michael Beitzel, (8/26/2019, 10:03 AM)

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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