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September 4 - September 10, 2016

What surface treatment is best when the maximum manufacturer recommended interval for overcoating has been exceeded?

Selected Answers

From MARIANA HUHULEA of Seaquest MPM on September 20, 2016:
Per, you are right. I have seen, however,  both sides of scenario: the disaster of a failed sweep blasting, which ended up with a full coating removal, but also a few situations where people ( Contractor) had the know-how  (angled blasting nozzles with remote control etc.) and they did it. But I must agree with you again that coating suppliers are sometimes looking to the other side when this issue pops-up. Proactive actions to avoid the problem should be a priority.

From Per Gabrielsson of Free Lance Consultations and Inspections on September 20, 2016:
Mariana, you just cannot sweepblast a ballast tank in a proper way due to the steel structures and small compartments, as well as web frames, stiffeners and brackets. When the overcoating window has been reached, there actually is no other safe way than to perform a full reblast. Honestly, a full reblast as a final cleaning would be much faster than a doubtful sweep blast as a final cleaning. Much depends also on the coating supplier, as some of those just would close their eyes and ears if the overcoating interval has been exceeded.

From Amitash Kumar of Akzonobel India Ltd on September 20, 2016:
For most coatings which exceed overcoat intervals, conduct secondary surface prepared by water washing followed by abrading the surface with hand-tool or power-tool cleaning, and then apply a surface-tolerant mastic coating.

From MARIANA HUHULEA of Seaquest MPM on September 18, 2016:
In shipbuilding, I would say, this is the worst-case scenario. For an expired coating in a water ballast tank, power-tool roughening would be in most of situations the best option among just a few alternative solutions. Special attention must be paid to edges and manual weld seams where power tools cannot entirely reach, and manual corrections by coarse sandpaper are a must. Dust collectors are highly recommendable for the job. (Even so, when the job is completed, the tank looks like a flour can with people dipped into it.) Ventilation adjustment during its vacuum cleaning is also very important. Personally, I am in full favor of garnet sweep blasting for its effectiveness and lesser delays; however, the risk of failure is higher if the contractor does not have the understanding and skill for a good sweep blasting.

From Luís Bonifácio of Omegacis, Unipessoal Lda. on September 15, 2016:
We had that problem in a new railway bridge which was sent to sit with primer and intermediate applied, the topcoat to be applied after erection. Due to delays the overcoating time was exceeded and the manufacturer's recommendation was to hydroblast at 500 bar (7300 psi) and apply the topcoat. The test with pull-off adhesion tests made (> 20) 3 months after application revealed values over 12 MPa (1740 psi). One year has passed and the result is still OK.

We shall abrade surface properly with 60 or 80 no. sand paper and apply an additional coat. Sweep blasting may damage the surface and the paint film can be contaminated. We can apply a bonding coat if it is recommended by paint manufacturer.

From Car F. of Municipal City on September 8, 2016:
I would roughen the surface and apply a bonding coat. It would probably be less expensive than risking coating failure in places where roughening wasn’t possible. A mistake like that would probably happens only once; then the lesson is learned.

From John Fauth of ChemMasters, Inc. on September 7, 2016:
Consult the manufacturer's technical department for the recommended surface treatment or removal instructions.

From Per Gabrielsson of Free Lance Consultations and Inspections on September 7, 2016:
Carry out careful manual roughening of the surface, but avoid scratching the surface. Sweepblasting may damage the structure of the coating to be overcoated and as this may not be detected, the entire job may be spoiled.

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Tagged categories: Asia Pacific; EMEA (Europe, Middle East and Africa); Latin America; North America; Overcoating

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