BlastTalk: Spot Abrasive Blasting and the right Abrasives
The repair of damaged coated steel surfaces is a difficult and time-consuming task. Many of these difficulties stem from the repair methods used and many asset owners take these repairs for granted.
They opt to use sub-standard repair methods such as chipping or mechanical power-tooling due to accessibility and perceived time, material, labor and equipment savings.
These sub-standard methods only partially remove corrosion products and damaged coatings which in turn, eventually results in premature failure to the touch-up coatings that were applied over these repair areas. Consequently, this results in the process having to be repeated again and again.
When the initial coating system was applied over the original surfaces, abrasive blasting would be the most preferred method and these substrates would be cleaned to White (Class Sa 3) or Near-White (Class 2½). If spot repairs are necessary, it is important that corrosion products in the coating breaks must be completely removed and the pit is corrosion free.
It is also very important that the damaged coating be removed to the point where it is solid, intact and tightly adhered to the surface. Otherwise, failure will continue to occur at the same spot.
It is therefore essential that the coating be tapered from the corrosion repair area out to the solid adhered area creating a featheredge at the junction between the steel and the old coating. This transition zone is not only necessary for a proper repair but also provides a much better appearance since there are no jagged rough edges to give the appearance of a patch job.
Therefore, Spot Abrasive blasting is the ideal repair method to address these concerns as it can effectively clean the corroded areas to the highest-class standards and create the interface between the blasted area and old coating edge.
Spot blasting has also been included in many specifications where new project coating systems need to be repaired due to erection damages.
To achieve optimal performance, the spot blasting procedure outlined in these specifications is to spot blast the rusty/damaged areas to minimum Class 2½, feather the intact coating edges, and light sweep blast to abrade the rest of the intact coating to remove dirt, chalk and other contaminants.
In numerous instances, asset owners may also wish to apply a full new coat over the intact good older areas just for aesthetic reasons. In these situations, lightly abrading the existing intact coating with a finer mesh sized abrasive such as GMA SpeedBlast™ garnet or Newsteel™ garnet, will provide sufficient mechanical bond capability for the next fresh paint layer.
|To round up the main procedure in a spot blasting process, the following key points are recommended:
- Use a predominately finer sized abrasive, such as 60 or 80 mesh so as to minimize the impact of larger particles on surrounding intact coatings. This also known as over-blast and may result in micro fractures in the coating film.
- Blast the corroded areas to the specified standard by normal operator methods. Holding the blast nozzle close-up and perpendicular allows for the larger and heavier abrasive particles to break up heavy rusts and damaged coated areas.
- Following this, back off the blast nozzle to between 1 to 1.3m (3 or 4 ft) and feather the intact coating edge at an oblique angle to the surface.
- Clean and abrade the solid coating surrounding the repair, continue to use the finer mesh abrasive by sweeping the existing coating both at a distance and at an oblique angle to prevent direct impact from the larger particles.
By Saravanan Madavamani, GMA Asia Pacific
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