Surface Preparation of Metals – Visual Cleanliness - Tips and Tricks:

NOTE: The following comments are the opinion of the author based on experience using the following equipment in both a laboratory and field environments. Any comments are welcome and can be forwarded to me at tswan@cci-inspection.com

Surface preparation is probably the main cause of most paint failures because more than any other factor, it affects how well the paint sticks to the surface being painted. Surface preparation can be broken down into two main categories.

• Surface Profile
• Surface Cleanliness

Surface profile is the determination of the roughness of the surface and for painting purposes involves depth of the profile and angularity of the profile. . More on Surface cleanliness can be found in the Surface Preparation – Profile - Tips and Tricks on the menu.

Surface Cleanliness involves determining how much of the original mill scale, rust and paint have been removed from the surface as well as how much invisible surface contamination is present usually in the form of salts. More on Surface cleanliness can be found in the Surface Preparation – Salts - Tips and Tricks on the menu.

Visual Cleanliness

Pint will bond to the surface being coated and if the surface is loose (rust, mill scale or old paint), when the surface breaks off, so will the paint. Some paints are formulated to coat over these surfaces with minimal surface preparation, but they should be used with caution and understanding.

Because there is not specific test for Visual Cleanliness, standards have been developed to determine specific levels of cleanliness. The most common visual standards are SSPC/NACE and ISO. Since the determination is visual, guides have been established to help clarify the text in the specifications. A brief summary follows.

SSPC-SP 1 – Solvent Cleaning

This specification covers the requirements for the solvent cleaning of steel surfaces. Removal of all detrimental foreign matter such as oil, grease, dirt, soil, salts, drawing and cutting compounds, and other contaminants from steel surfaces by the use of solvents, emulsions, cleaning compounds, steam or other similar materials and methods which involve a solvent or cleaning action.

SSPC-SP 2 - Hand Tool Cleaning

This specification covers the requirements for the hand tool cleaning of steel surfaces. Removal of all rust scale, mill scale, loose rust and loose paint to the degree specified by hand wire brushing, hand sanding, hand scraping, hand chipping or other hand impact tools or by a combination of these methods. The substrate should have a faint metallic sheen and also be free of oil, grease, dust, soil, salts and other contaminants.

SSPC-SP 3- Power Tool Cleaning

Specifies the use of power assisted hand tools to obtain a steel surface free of all loose mill scale, loose rust, loose paint (although in ZRC's case, we require complete removal of all paint), and other loose detrimental foreign matter. It is not intended that adherent mill scale and rust be removed by this process. Mill scale and rust are considered adherent if they cannot be removed by lifting with a dull putty knife

.SSPC-SP 5/NACE No. 1 - White Metal Blast Cleaning

This standard covers the requirements for white metal blast cleaning of steel surfaces by the use of abrasives. Removal of all mill scale, rust, rust scale, paint or foreign matter by the use of abrasives propelled through nozzles or by centrifugal wheels. A White Metal Blast Cleaned Surface Finish is defined as a surface with a gray-white, uniform metallic color, slightly roughened to form a suitable anchor pattern for coatings. The surface, when viewed without magnification, shall be free of all oil, grease, dirt, visible mill scale, rust, corrosion products, oxides, paint, or any other foreign matter.

SSPC-SP 6/NACE No. 3 ISO 8501 1-1: 1988(E) (SIS 05 59 00) Sa 2 - Commercial Blast Cleaning
This standard covers the requirements for commercial blast cleaning of steel surfaces by the use of abrasives. Removal of mill scale, rust, rust scale, paint or foreign matter by the use of abrasives propelled through nozzles or by centrifugal wheels, to the degree specified. A commercial blast cleaned surface finish is defined as one from which all oil, grease, dirt, rust scale and foreign matter have been completely removed from the surface and all rust, mill scale and old paint have been completely removed except for slight shadows, streaks, or discolorations caused by rust stain, mill scale oxides or slight, tight residues of paint or coating that may remain; if the surface is pitted, slight residues of rust or paint may by found in the bottom of pits; at least two-thirds of each square inch of surface area shall be free of all visible residues and the remainder shall be limited to the light discoloration, slight staining or tight residues mentioned above.

SSPC-SP 7/NACE No. 4 -Brush-Off Blast Cleaning

This standard covers the requirements for brush-off blast cleaning of steel surfaces by the use of abrasives. Removal of loose mill scale, loose rust, and loose paint, to the degree hereafter specified, by the impact of abrasives propelled through nozzles or by centrifugal wheels. It is not intended that the surface shall be free of all mill scale, rust, and paint. The remaining mill scale, rust, and paint should be tight and the surface should be sufficiently abraded to provide good adhesion and bonding of paint. A brush-off blast cleaned surface finish is defined as one from which all oil, grease, dirt, rust scale, loose mill scale, loose rust and loose paint or coatings are removed completely but tight mill scale and tightly adhered rust, paint and coatings are permitted to remain provided that all mill scale and rust have been exposed to the abrasive blast pattern sufficiently to expose numerous flecks of the underlying metal fairly uniformly distributed over the entire surface.

This standard covers the requirements for Near-White Metal Blast Cleaning of steel surfaces by the use of abrasives. Removal of nearly all mill scale, rust, rust scale, paint, or foreign matter by the use of abrasives propelled through nozzles or by centrifugal wheels, to the degree hereafter specified. A Near-White Blast Cleaned Surface Finish is defined as one from which all oil, grease, dirt, mill scale, rust, corrosion products, oxides, paint or other foreign matter have been completely removed from the surface except for very light shadows, very slight streaks or slight discolorations caused by rust stain, mill scale oxides, or light, tight residues of paint or coating that may remain. At least 95 percent of each square inch of surface area shall be free of all visible residues, and the remainder shall be limited to the light discoloration mentioned above.

SSPC-SP 11 - Power Tool Cleaning to Bare Metal –

Specifies the use of power tools to produce a bare metal surface and to retain or produce a surface profile. This specification is suitable where a roughened, clean, bare metal surface is required, but where abrasive blasting is not feasible or permissible. Once cleaned, the surface will be free of visible oil, grease, dirt, dust, mill scale, rust, paint, oxide, corrosion products, and other foreign matter. Slight residue of rust and paint may be left in the lower portion of pits if the original surface is pitted. Surface shall have a degree of roughness (profile) of no less than 1 mil (25 microns).

SSPC-SP 12/NACE No. 5: Surface Preparation and Cleaning of Steel and Other Hard Materials by High-and Ultrahigh-Pressure Water Jetting Prior to Recoating

This standard provides requirements for the use of high- and ultrahigh pressure water jetting to achieve various degrees of surface cleanliness. This standard is limited in scope to the use of water only without the addition of solid particles in the stream.

SSPC-SP 13/NACE No. 6 - Surface Preparation of Concrete

This standard gives requirements for surface preparation of concrete by mechanical, chemical, or thermal methods prior to the application of bonded protective coating or lining systems. The requirements of this standard are applicable to all types of cementitious surfaces including cast-in-place concrete floors and walls, precast slabs, masonry walls and shotcrete surfaces.

An acceptable prepared concrete surface should be free of contaminants, laitance, loosely adhering concrete, and dust, and should provide a dry, sound, uniform substrate suitable for the application of protective coating or lining systems. Depending upon the desired finish and system, a block filler may be required.

SSPC-SP 14/NACE No. 8 – Industrial Blast Cleaning

This joint standard covers the use of blast cleaning abrasives to achieve a defined degree of cleaning of steel surfaces prior to the application of a protective coating or lining system. Industrial blast cleaning provides a greater degree of cleaning than brush-off blast cleaning (NACE No. 4/SSPC-SP 7), but less than commercial blast cleaning (NACE No. 3/SSPC-SP 6). Industrial blast cleaning is used when the objective is to remove most of the coating, mill scale, and rust, but when the extra effort required to remove every trace of these is determined to be unwarranted.

The difference between an industrial blast and a brush-off blast is that the objective of a brush-off blast is to allow as much of an existing coating to remain as possible, while the purpose of the industrial blast is to remove most of the coating.

A commercial blast is free of mill scale, rust, and coatings, and allows only random staining on less than 33% of the surface. The industrial blast allows defined mill scale, coating, and rust to remain on less than 10% of the surface and allows defined stains to remain on all surfaces.

SSPC-SP 15 Commercial Grade Power Tool Cleaning

This standard covers the requirements for power tool cleaning to provide a commercial grade power tool cleaned steel surface, and to retain or produce a minimum 25 micrometer (1.0 mil) surface profile. A commercial grade power tool cleaned steel surface, when viewed without magnification, shall be free of all visible oil, grease, dirt, rust, coating, oxides, mill scale, corrosion products, and other foreign matter, except as noted.

Random staining shall be limited to no more than 33 percent of each unit area of surface as defined. Staining may consist of light shadows, slight streaks, or minor discolorations caused by stains of rust, stains of mill scale, or stains of previously applied coating. Slight residues of rust and paint may also be left in the bottoms of pits if the original surface is pitted.

This standard differs from SSPC-SP 3, Power Tool Cleaning, in that a higher degree of surface cleanliness is required, and a minimum surface profile of 25 micrometers (1.0 mil) will be retained or produced.

This standard differs from SSPC-SP 11, Power Tool Cleaning to Bare Metal, in that stains of rust, paint, or mill scale may remain on the surface.

SSPC VISUAL STANDARDS

It is important to understand that the Guides only describes the pictorial standard and does not constitute the standard. It is to be used for comparative purposes and is not intended to have a direct relationship to a decision regarding painting requirements.

SSPC/NACE/ISO/Swedish Standards Chart

I have a Specification that call for SSPC SP-X/NACE Y, what is my first step.

If you have a specification that requires abrasive blast , Hand tool, Power Tool or WaterJetting, Make sure you have a copy of the standard specified. Unless otherwise stated in the specification, always use the most current version. If there is a Vis Guide applicable to the method being used, it is useful to have these on site. These Can be Purchased from the specifying organizations or from CCI.

I have a Specification that requires SSPS SP6/NACE 3, do I need to do SSPC SP1?

YES. All the surface preparation methods require removal of oil. Grease and other contaminants by SSPC-SP1 or other agreed upon method. If you do not clean the surface first, you do not meet the specification criteria.

The contractor says all the mill scale has been removed, but I think I still see some?

Mill scale usually fractures and breaks off during blasting and does not accept a profile. If there is any question, apply a couple of drops of 5% Copper Sulfate to the area in question, Steel will turn copper colored and mill scale will not.

I am arguing with the contractor over “staining” verses “rust”?

Rust is iron oxide that is attached to the surface of the steel and must be removed. If you have ever tried to get rust out of a shirt, you know that it can stain and does not come out easily. This can also happen to steel. A stain is part of the metal and can only be removed by removing the surface of the steel.

Think of it like wearing a white shirt is the metal. You spill spaghetti sauce on your shirt, it is like rust. Remove the sauce by rubbing with a wet cloth and you still have a red stain. It is OK to have the sauce stain just not the sauce.

As a last resort, you can examine the surface under magnification and determine if the stain is part of the surface or above the surface. The Elcometer 7210 50X magnifier works great for this. (Note – the method states “viewed without magnification” so use caution when using magnification for what you are looking at)

I have a project that requires SSPC SP2 or SP3. Before I start, I can not remove any rust, mill scale or paint with a Dull Putty Knife. Am I done?

NO. The dull putty knife test applies after the hand tool or power tool cleaning Once the entire surface is clean, then check the surface with the dull putty knife.

Using Vis 1, the contractor says he has met the spec, I say he has not.

Remember, Vis 1 (and the other Vis guides) is a guide to help explain the standard. In case of disputes, the text of the method determines if the surface had met the specification. This being said, there are many factors that affect the appearance of the final blasted surface. Make sure you are using the proper starting grade for the metal in the Vis Guide.

Also, in the back if the Vis 1 Guide, it shows metal blasted to an SP5 with several different media. These all meet the criteria of an SP5 but they all have a different appearance.

My specification calls to use VIS 1 to determine surface profile. How do I do that?

You can’t. Surface profile has nothing to do with surface cleanliness. Prior to doing any work, this needs to be addressed and you may want to refer them to CCI Inspection Consulting Services since they should not be writing specifications.

Do I really need a “white metal” blast or is “near white” OK?

In general, you need a white metal blast for immersion surfaces, metalizing, and inorganic zinc. For most other services, near white is usually OK. The coating manufacturer should have the final determination of the required cleanliness.

Also, on new steel, there should not be any difference between a white metal and a near white blast because there should not be any staining.

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