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July 18 - July 24, 2011

What is the remedial action if a blaster produces too high a surface profile for the required primer thickness?

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Selected Answers

From jesse chasteen of schriener construction on July 22, 2011:

     Either way, it requires rework. As Chuck questioned where was the surface profile at on the test plate? What was the spec on the abrasive? If the manufacturer stated that a specific profile range at a certain PSI at the nozzle would result in the appropriate profile  and it didn't, then the rework should be subsidized.

     Many questions as to why the excessive profile was created must be looked at to ensure that a repeat performance doesn't happen. If it was an out of spec pre-existing profile issue, the change to the abrasive after inspection of the test plate would have been grounds for a change order and the contractor would have appeared to be more professional. Always put the horse in front of the cart; it's the best way to move forward.

From mark young of kentz on July 22, 2011:
     Sweep blast will be fine, but making sure the angle of the nozzle is directed to smooth away the peaks of the profile. A fine abrasive or garnet should be sufficient. Obviously, the wrong abrasive was used in the first place.

From Sukhmander Singh of Serv Consult on July 21, 2011:
    The surface has a higher profile than specified for the primer coat; primer paint is not yet applied. Re-blasting with the same abrasive size can break away the peaks and reduce the profile.

From richard d souza of stoncor middle east llc on July 21, 2011:
     When such a case arises, people re-blast with a fine grade sand or garnet abrasive to try to reduce the profile, but it is always assumed that re-blast with a finer abrasive will bring down the profile into the desired profile range. But I am very sure that, at best, you may be able to bring the profile down by  10-15 microns by knocking off the high profile peaks, and it almost impossible to change a 100-micron profile to a 50-micron profile with this technique. The same problem exists if there is an excessive pre-existing profile under the existing coating being removed and if there is existing rust pitting on the surface. In such case, all parties involved must reach an agreement and take appropriate steps to account for such variables, changing the coating system  and/or increasing the total dft to meet or exceed the specification requirements.

From Chuck Stevens of self on July 20, 2011:
     Check the condition of the steel. Determine if the ambient temps are are within specs. Apply inorganic zinc as per manufacturer specs.prior to intermediate coat. (Why was this allowed to happens? Where was the QC person?What happened to the blast sample as agreed to by the parties involved?) This should not have happened. It would not have happened on my shift.

From tarun datta of Berger Paints Bahrain on July 20, 2011:
     Reblast the surface with fine abrasives or apply high build primer, if allowed.

From Barry Barman of Barry Barman & Associates on July 19, 2011:
    Reblast using a finer abrasive. The finer abrasive will result in the cold flow of surface steel creating a greater density of peaks and valleys per unit of surface area but with a shallower profile.

From Venkatasubramanian Chandramouly of Berger Paints Emirates Limited on July 18, 2011:
     Then the easiest remedy is apply blast profile + 20 microns more of the primer if allowed by the manufacturer. Otherwise, we recommend to apply a 25 micron, inorganic zinc shop primer and after proper curing apply the usual coating.

From mangai kumar of insignia on July 18, 2011:
     I would suggest to reduce the nozzle pressure and sweep blast with fine abrasives.

From Carl Havemann of www.corrosioneducation.co.za on July 18, 2011:
     Check that the measurements are statistically representative. Use emery paper of suitable roughness to abrade the surface to reduce high points. Establish this method by experiment. This may only be suitable for small areas, however.  Or replace the primer with one having a higher film build to accommodate the profile.

From bryan buckley of corcon inc. on September 21, 2011:
     Reblast with the same grit; just run the hose on the heavy side (almost chugging), and your peaks will be knocked down.

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Tagged categories: Abrasive blasting; Surface preparation; Surface profile


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