Repair, remediation and maintenance coating activities can loosely be grouped into four classifications:

• Spot Blasting: During fabric maintenance works, spot repair should be considered if the coated area has localized (spot) rusting only and the remaining paint film is sound, tightly adhered and in good condition.
• Spot and Sweep dry abrasive blast cleaning: Refurbishment should be considered when the coated area has greater localized (spot) rusting while the adjacent coating is largely sound.
• Major Maintenance
• Temporary Coating.

Let’s have a look at Spot Blasting and Sweep Blasting coating repairs and the necessity of achieving suitable Surface preparation.

Repairing coated steel surfaces that have sustained damage is an intricate and often labor-intensive endeavor.

The choice of repair techniques is critical for each of these classifications, yet it is common to see asset owners opting for seemingly cost-effective but inferior methods like chipping or mechanical power-tooling. These approaches, while accessible and initially perceived as economical, fall short of thoroughly removing corrosion and compromised coatings. This shortfall leads to the early deterioration of newly applied touch-up coatings, necessitating repeated repair efforts. It is widely recognized in the Corrosion Industry that 75% of coating failures can be attributed to inadequate surface preparation.

SSPC standard SSPC -SP-18 contains the requirement for “spot and sweep” dry abrasive blast cleaning a previously coated carbon steel surface.

This technique can be an economical way to maintain existing coatings which have some serviceable remaining life. The intent is to remove all unserviceable coating, clean any exposed steel to a near-white metal cleanliness, and uniformly roughen the remaining serviceable coating.

The practice is not new, but SSPC-SP 18 is the first industry standard to comprehensively describe the practice. 

Thorough spot and sweep blast cleaning is designed to subject the entire surface being prepared to significant abrasive impact. Coating that cannot withstand the abrasive blasting process is removed, while the remaining existing coating is suitably prepared for an additional layer of coating. Areas of exposed steel are prepared to the level of near-white metal blast cleaning. 

When originally applying coating systems to steel surfaces, abrasive blasting stands out as the preferred method, achieving thorough cleanliness to standards such as White (Class Sa 3) or Near-White (Class 2½). This level of cleanliness is equally crucial for effective spot repairs, ensuring the complete removal of corrosion and the stripping of damaged coatings down to a robust, securely adhered base layer. Overlooking this step results in persistent problems in the affected areas.

Spot Blasting is also recommended in specifications for repairing coating systems on new projects that have incurred damage during construction and is described in SSPC-SP-18 section 2.2 as “Localized abrasive blast cleaning as used in surface preparation for maintenance painting. Often applied to specific areas where corrosion or coating weaknesses are evident (e.g., blistering, delamination, cracking)”

The following repair procedure should be applied at all locations where spot corrosion / damage has occurred:

1. The area of steel that requires coating repairs shall be dry and free from dirt, dust, oil and grease and loose and flaking paint or any other contamination that may compromise the performance of the coating system.
2. Abrasive blast cleaning of damaged areas should be conducted in a stop-start manner to ensure no over-blast damage is done to the intact coating. The substrate surface profile of the spot blasted area shall be suitable for the nominated coating system.
3. Masking of intact coating to prevent over-blast damage may be necessary.
4. Edges of the sound coating adjacent to the bare metal, cleaned surfaces shall be feathered back to a sound surface and roughened over a distance of 100 mm from the edge of the bare metal.
5. No loose or flaking paint shall remain around the damaged area.

An essential part of the repair process is creating a smooth gradient in the coating from the repaired site to the undamaged area, forming a featheredge that not only solidifies the repair but also improves its visual appeal by avoiding the appearance of rough, noticeable patches.

For aesthetic upgrades or complete resurfacing, a gentle “sweep” abrasion of the existing intact coating with finer abrasives and is described in SSPC-SP-18 section 2.3 as “A fast pass of the abrasive blasting pattern over a surface to remove loose material and to roughen the surface sufficiently to successfully accept a coat of paint.”

The following should be applied when Sweep Dry abrasive blasting:

1. All surfaces, including corroded areas and sound paint, shall be sweep blasted.
2. Use a fine garnet abrasive such GMA SpeedBlast™ or Newsteel™.
3. Blast from a distance of 450 to 550 mm from the painted surface, directed at an angle of no greater than 45° to the surface.

By employing a gentle, angled sweeping technique with finer abrasives prepares the surface for a new paint application by improving the mechanical bond. Adopting these techniques not only ensures the structural and visual quality of repairs but also underscores the strategic value of Spot Abrasive Blasting in the maintenance and aesthetic enhancement of steel surfaces.