Standards for Partial Coating Removal During Maintenance Painting

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Common methods of maintenance painting have either involved complete removal of existing paint (for example preparing to an SSPC-SP 10 / NACE No. 2, “Near-White Blast Cleaning”¹ surface) or a highly localized preparation of corroding areas using hand or power tools (SSPC-SP 2 or SSPC-SP 3). Sometimes projects may be designed to an intermediate degree where the intent is to reduce the level of surface preparation effort by allowing “good” coatings to remain. In these cases, abrasive blasting or ultra-high-pressure (UHP) waterjetting may be used to prepare the entire surface, but well-adhered, aged coating is permitted to remain. There are a number of standards which can be used to describe such work including SSPC-SP 14 / NACE No. 8, “Industrial Blast Cleaning,” SSPC-SP 7 / NACE No. 4, “Brush-Off Blast Cleaning,” SSPC-SP 6 (WAB) / NACE WAB-3 “Thorough Waterjetting” and SSPC-WJ 4 / NACE WJ-4, “Light Waterjetting.” Concepts associated with maintenance painting are covered in a number of documents including SSPC-PA 1, “Shop, Field and Maintenance Painting of Steel”² and SSPC-PA 4, “Guide to Maintenance Repainting with Oil Base or Alkyd Painting Systems.”³ This article will review these standards and discuss common issues associated with their use.

As seen in the July 2017 Edition of the Journal of Protective Coatings and Linings (JPCL) & Paintsquare.com

The concept of overcoating is discussed in SSPC Technology Update SSPC- TU 3, “Overcoating.”⁴ The document defines overcoating as “application of coating materials over an existing coating in order to extend its service life, including use of the appropriate cleaning methods.” In most cases, the procedure includes preparation of rusted or degraded areas, feathering edges of existing paint, cleaning of the remaining coating, application of a primer coat over repaired areas, and application of a full coat over the entire structure. Intermediate coats are sometimes applied over primed areas or over the entire structure. Overcoating may be a cost-effective alternative to complete coating removal and repainting, but it can also present a risk of premature failure.

STANDARDS FOR PARTIAL COATING REMOVAL

It is necessary to establish a meaningful description of “partial” coating removal work for several reasons. Contract specifications must ensure that the owner and contractor have a common understanding of the work to be performed, supervisors must communicate procedures to workers and inspectors must have meaningful and enforceable inspection criteria.

A subtle but important distinction for partial coating removal is the difference between spot-blasting and sweep-blasting. Spot-blasting involves localized abrasive-blast procedures to clean specific spots where corrosion or coating failures are evident. Sweep-blasting refers to a fast pass of the abrasive blasting pattern over a surface to remove loose material and to roughen the surface sufficiently so that it will successfully accept a coat of paint. Most maintenance projects with partial coating removal will involve some degree of spot- and sweep-blasting.

In the United States, industrial painting projects typically refer to surface preparation standards developed by SSPC and NACE. Surface preparation standards which may be used to describe partial coating removal using abrasive-blasting methods are listed in Table 1. Surface preparation standards which may be used to describe partial coating removal using waterjetting methods are listed in Table 2. Both tables also contain text from the standards and a representative photograph of a prepared surface from SSPC-VIS 1, “Guide and Reference Photographs for Steel Surfaces Prepared by Dry Abrasive Blast Cleaning”⁵ or SSPC-VIS 4, “Guide and Reference Photographs for Steel Surfaces Prepared by Waterjetting.”⁶ Any of these standards could be used for partial coating removal; there does not seem to be any consensus on a single, ideal standard.

Table 1: Relevant Abrasive Blasting Surface Preparation Standards.

Tables courtesy of SSPC

Table 2: Relevant Waterjetting Surface Preparation Standards.

SSPC-SP 7 / NACE No. 4, “Brush-Off Blast Cleaning”⁷ is an abrasive-blasting standard which allows tightly adherent mill scale, rust and coating to remain on the surface. There is no requirement to expose any bare metal. While the specification requires that the entire surface be subjected to the abrasive blast, the standard doesn’t provide objective criteria that an inspector could use to determine whether the surface was sufficiently subjected to the abrasive blast. Angle of attack, dwell time, standoff distance and media size can all impact the degree to which a coating is roughened and/or removed.

By contrast, SSPC-SP 14 / NACE No. 8, “Industrial Blast Cleaning”⁸ only allows traces of tightly adherent mill scale, rust and coating residues to remain on 10 percent of each unit area. Areas of limited accessibility for blast-cleaning such as crevices and tight configurations are exempt from the surface-area requirement but must still be subjected to the abrasive blast. The area requirement in this standard makes it somewhat easier for an inspector to establish conformance with the specification.

Waterjet Cleaning of Metals SSPC-SP 6 (WAB) / NACE WAB-3 “Thorough Waterjetting”⁹ also limits the extent of allowable tightly adherent mill scale, rust and coating residues. WJ-3 is used to remove much of the rust and other corrosion products, coating, and mill scale, and leave tightly adherent thin films on less than 33 percent of each unit area.

Waterjet Cleaning of Metals SSPC-SP 7 (WAB) / NACE WAB-4 “Light Waterjetting”¹⁰ allows as much as possible of the tightly adherent existing coating or tightly adherent foreign matter to remain. The specification requires that the entire surface be subjected to waterjet cleaning. However, as with abrasive techniques, there is no objective criteria that an inspector can use to determine whether the surface was sufficiently subjected to waterjetting. Here again, angle of attack, dwell time and standoff distance can all impact the degree to which loose coating is removed.

TECHNICAL CHALLENGES ASSOCIATED WITH PARTIAL COATING REMOVAL

By definition, a partial blast process will allow coating to remain on the prepared surface. Project personnel including blasters and inspectors must to be able to determine when the remaining coating is acceptable. The characteristics used to describe the remaining coating might involve how thick it is, how rough it is and whether or not it is tightly adherent.

Most surface preparation standards for partial coating removal allow stains of previously applied coating to remain; presumably any detectable thickness of remaining coating would be more than a stain.

The four specifications for partial coating removal each allow tightly adhering rust, mill scale and coating to remain on the surface. The waterjetting standards characterize the allowable remaining coating as “thin” while the abrasive-blast standards make no distinction based on thickness. Standard techniques for determining dry-film thickness can be used on any remaining coating. While remaining coating thickness is sometimes measured, the data is typically for reference only and not part of any accept/reject criteria.

SSPC-SP 14 / NACE No. 8 and SSPC-SP 6 (WAB) / NACE WAB-3 both require that the remaining coating is limited and evenly distributed, which in effect ensures that the entire structure surface is subjected to the cleaning methods. SSPC-SP 7 / NACE No. 4 and SSPC-SP WJ-4 / NACE WJ-4 require that the entire surface is subjected to the method and that all loose material is removed. Roughness of the remaining coating can be measured with profilometers or replica tape. However, simply looking for visual evidence that the remaining coating has been subjected to the cleaning method being used is typically adequate.

Because the standards only allow tightly adhering material to remain, coating professionals must have the ability to determine if the remaining coating is tightly adherent. The industry standard technique of probing with a dull putty knife is almost universally employed. SSPC defines putty knife for inspection as a commercially manufactured metal blade which has a width of 1.5-to-3 inches, a length of 3-to-5 inches, a thickness of 30-to-50 mils (which is not worn to less than 75 percent of its original thickness) and whose edge is in good condition (not nicked, gouged or contaminated with dry paint). When inspecting for lose adherent coating, the blade must be held at a maximum of 45 degrees to the surface and the corners of the blade shall not be used to dig the coating or residues. Personnel responsible for the quality of the work should be familiar with the nuances of using a dull putty knife for coating adhesion.

COST SAVINGS

The concept of partial surface preparation is supported with qualitative logic such as “fix only what is broken” or “stop removing good paint.” Obviously there is a reduction in the labor required and waste generated if less effort is put into removing old paint from a surface.

Unfortunately, it is difficult to capture the potential for savings on any specific job because the savings are dependent on how the existing coating responds to the surface preparation method, i.e., the condition of the coating.

Anecdotally, it is reported that a disproportionate effort is spent removing the last traces of paint in an effort to achieve an SSPC SP-10, near-white blasted surface. Rework, which requires multiple iterations of cleaning, inspection and re-blasting, will have a higher cost-per-unit area than the originally performed work. No published studies have captured or quantified the incremental cost of the last elements of work when attempting to achieve an SP-10, near-white blasted surface.

There have been studies and reports which compare the relative cost of achieving an SSPC-SP 10, near-white blasted surface with an SSPC-SP 7, brush-off blast-cleaned surface. A study on retaining pre-construction primer reported a 50-percent reduction in the labor required to attain an SSPC-SP 7 surface versus an SSPC-SP 10 surface for pre-construction-primed tanks being blasted in a blast hall.¹¹ Similarly, MeLampy, et. al, reported that the average cost of an SSPC-SP 7, brush-off blast-cleaned surface preparation is 52 percent of the cost of an SSPC-SP 10, near-white blasted surface.¹² A study on spot and sweep preparation of underwater hulls using a waterjetting robot reported that the spot and sweep process was about 33 percent of the full removal cost.¹³

EXPECTED SERVICE LIFE

A final issue that must be addressed concerning partial surface preparation schemes is the expected service life of the coated surface. This is essential to establishing a business case for partial surface preparation versus complete removal of the aged coating. A partial preparation scheme may result in an average expected service life that is shorter than a new coating system, increase the risk of premature failure, or both.

Many owners of industrial structures have established service-life expectations for their coating systems. For most industrial structures with long (10-to-20-year) service lives, the expectations are based in large part on extensive field experience with similar products.

Maintenance painting with partial coating removal may be performed for aesthetic reasons or in conjunction with other work requiring coating removal — for example, structural repairs. In these instances, the installed cost typically drives the decision-making. However, once the coatings reach the end of their expected service lives, there is little data to suggest whether a new coating system installed over intact old coating will actually achieve the expected design service life. The belief that a new coating system will only achieve its design service life over a clean metal surface discourages owners from incorporating partial coating removal into coating replacement projects.

While partial coating removal may have lower short-term costs, it also introduces a risk of failure if poor coating is left on the surface. The concept of increased risk is discussed in a long term study of bridge overcoating.¹⁴ By defining a success metric, risk could be determined based on the probability of success associated with various coating/surface preparation scenarios. The risk data could then be combined with cost data to identify the best value coating strategy.

CONCLUSIONS

Partial surface preparation is a cost-saving concept with merit and demonstrated success in low-risk applications. Two key challenges must be overcome prior to adopting partial surface preparation for high-risk applications with long desired service life.

A consensus must be reached on how to use the appropriate technical standards and processes to be employed for partial surface preparation. Data must be generated which supports the business case for partial surface preparation including the potential for cost and schedule savings as well as any increased performance risk or shortened service life.

ABOUT THE AUTHORS

J. Peter Ault has been actively involved in various aspects of corrosion control and materials engineering for over 25 years. Since 2006, he has been a principal of Elzly Technology Corporation. Ault is an active member of several technical societies including SSPC, ASTM, NACE International, ASNE, SNAME and NPSE. He is a registered professional engineer in New York and New Jersey and holds coatings specialist certifications from both SSPC and NACE. Ault has a B.S. degree in mechanical engineering and an MBA from Drexel University.

Eric Shoyer has worked with Elzly Technology Corporation in various aspects of corrosion control and materials engineering for over eight years. He is an active member of several technical societies including SSPC and NACE International and is a registered NACE CIP Level 2. Shoyer holds Bachelor of Science degrees in civil engineering and structural engineering from Drexel University.

REFERENCES

SSPC SP-10 / NACE No. 2, “Near-White Blast Cleaning,” Pittsburgh, Pa., SSPC and Houston, Texas, NACE.
SSPC: The Society for Protective Coatings, “Paint Application Guide No. 1: Shop, Field, and Maintenance Painting of Steel,” November 2004.
SSPC: The Society for Protective Coatings, “Paint Application Guide No. 4: Guide to Maintenance Repainting with Oil Base or Alkyd Painting Systems,” November 2004.
SSPC: The Society for Protective Coatings, “Technology Update No. 3: Overcoating,” November 2004.
SSPC-VIS 1, “Guide and Reference Photographs for Steel Surfaces Prepared by Dry Abrasive Blast Cleaning,” SSPC Publication 02-12, Pittsburgh, Pa.
SSPC-VIS 4 / NACE VIS 7, “Guide and Reference Photographs for Steel Surfaces Prepared by Waterjetting,” Pittsburgh, Pa: SSPC and Houston, Texas: NACE.
SSPC SP 7 / NACE No. 4, “Brush-Off Blast Cleaning,” Pittsburgh, Pa: SSPC and Houston, Texas: NACE.
SSPC SP 14 / NACE No. 8, “Industrial Blast Cleaning,” Pittsburgh, Pa: SSPC and Houston, Texas: NACE.
SSPC-SP 6 (WAB) / NACE WAB-3 “Thorough Waterjetting,” Pittsburgh, Pa: SSPC and Houston, Texas: NACE.
SSPC-SP 7 (WAB) / NACE WAB-4 – “Light Waterjetting,” Pittsburgh, Pa: SSPC and Houston, Texas: NACE.
“Retention of Pre-Construction Primer,” NSRP Surface Preparation and Coatings Panel Project Report, November, 2010.
MeLampy, et.al., “Costing Considerations For Maintenance and New Construction Coating Work,” Paper No. 509, NACE International, Corrosion 1998.
“Evaluation of Spot and Sweep Blasting as a Cost-Effective Method of Underwater and Outer Hull Surface Preparation,”National Shipbuilding Research Program (NSRP) Surface Preparation and Coatings Panel (SPC), Gordon Kuljian, et al, May 2012.
Ault and Farschon, “20-Year Performance of Bridge Maintenance Systems,” PACE 2008.

STANDARD	DESCRIPTION (EMPHASIS ADDED)	SSPC VIS 1 PHOTOGRAPH
SSPC-SP 7 / NACE No. 4 – Brush-Off Blast Cleaning⁷	A brush-off blast cleaned surface, when viewed without magnification, shall be free of all visible oil, grease, dirt, dust, loose mill scale, loose rust and loose coating. Tightly adherent mill scale, rust and coating may remain on the surface. Mill scale, rust and coating are considered tightly adherent if they cannot be removed by lifting with a dull putty knife after abrasive blast cleaning has been performed. Note that the entire surface shall be subjected to the abrasive blast.
SSPC-SP 14 / NACE No. 8 – Industrial Blast Cleaning⁸	An industrial blast cleaned surface, when viewed without magnification, shall be free of all visible oil, grease, dust and dirt. Traces of tightly adherent mill scale, rust, and coating residues are permitted to remain on 10 percent of each unit area of the surface (approximately 5,800 mm² [9.0 in.²]) (i.e., a square 76 mm x 76 mm [3.0 in. x 3.0 in.]) if they are evenly distributed. The traces of mill scale, rust and coating are considered to be tightly adherent if they cannot be lifted with a dull putty knife. Shadows, streaks and discolorations caused by stains of rust, stains of mill scale and stains of previously applied coating may be present on the remainder of the surface.

STANDARD	DESCRIPTION (EMPHASIS ADDED)	SSPC VIS 4 / NACE VIS 7 PHOTOGRAPH
Waterjet Cleaning of Metals SSPC-SP 6 (WAB) / NACE WAB-3 – Thorough Cleaning⁹	A metal surface after Thorough Cleaning, when viewed without magnification, shall have a matte (dull, mottled) finish and shall be free of all visible oil, grease, dirt, rust and other corrosion products except for randomly dispersed stains of rust and other corrosion products, tightly adherent thin coatings, and other tightly adherent foreign matter. The staining or tightly adherent matter shall be limited to no more than 33 percent of each unit area of surface and may consist of randomly dispersed stains of rust and other corrosion products or previously applied coating, tightly adherent thin coatings, and other tightly adherent foreign matter.
Waterjet Cleaning of Metals SSPC-SP 7 (WAB) / NACE WAB-4 – Light Cleaning¹⁰	A metal surface after Light Cleaning, when viewed without magnification, shall be free of all visible oil, grease, dirt, dust, loose mill scale, loose rust and other corrosion products, and loose coating. Any residual material shall be tightly adhered to the metal substrate and may consist of randomly dispersed stains of rust and other corrosion products or previously applied coating, tightly adherent thin coatings and other tightly adherent foreign matter.

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Standards Used for Partial Coating Removal During Maintenance Painting