SSPC-SP 18

December 30, 2020

SSPC: The Society for Protective Coatings

Thorough Spot and Sweep Blast Cleaning for Industrial Coating Maintenance

Surface Preparation Standard No. 18

Foreword

This SSPC standard contains requirements for dry abrasive blast cleaning a previously coated carbon steel surface to prepare retained coating for maintenance; and to clean areas of bare steel to near-white metal as defined in SSPC-SP 10/NACE No. 2. Thorough spot and sweep blast cleaning is used when the objective is to remove all unserviceable coating, clean any exposed steel to a near-white metal cleanliness, and uniformly roughen the remaining serviceable coating. 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. This standard is intended for use by coating or lining specifiers, applicators, inspectors, or others whose responsibility is to define a standard degree of surface cleanliness for previously coated carbon steel surfaces.

In this standard, the terms shall and must are used to state mandatory requirements. The term should is used to state something considered good and recommended but not mandatory. The term may is used to state something that is considered optional.

Scope

1.1 This standard contains the requirements for the

Thorough Spot and Sweep Blast Cleaning degree of visible surface cleanliness of previously coated steel surfaces by the use of dry abrasive blast cleaning. These requirements include the end condition of the surface as determined by visual inspection, and materials and procedures used to achieve and verify the end condition. Thorough spot and sweep blast cleaning is designed to subject the entire surface being prepared to abrasive impact. Coating that cannot withstand the abrasive blasting process is removed, while the remaining existing coating is suitably prepared for application of an additional layer of coating. Areas of exposed steel are prepared to the level of SSPC-SP 10/NACE No. 2.

1.2 This standard is limited to requirements for

visible surface contaminants. Information on nonvisible contamination is in nonmandatory Appendix A1. Information on soluble salt testing is in SSPC-Guide 15.

1.3 Information about the function of Thorough Spot

and Sweep Blast Cleaning is in Appendix A2. Appendix A3 contains existing coating considerations for project planners and managers.

1.4 Units of Measure: This standard provides both

IEEE/ASTM(1) SI 10 International System Units (SI) units and U.S. Customary units. SI Units are presented first, with a conversion into approximate U.S. custom units shown in parentheses. The conversions are not exact; therefore, each system must be used independently of the other.

2: Definitions

2.1 Thorough Spot and Sweep Blast Cleaned

Surface: A thorough spot and sweep blast cleaned surface, when viewed without magnification, shall consist of areas of exposed steel cleaned to near-white metal level as defined in Section 2.1.1, as well as areas of retained existing coating as described in Sections 2.1.2 and 2.1.3.

2.1.1 As defined in SSPC-SP 10/NACE No. 2, near-

white blast cleaned steel surfaces shall be free of all visible oil, grease, dust, dirt, mill scale, rust, coating, and other foreign matter. Random staining shall be limited to no more than 5 percent of each unit area of surface (approximately 5,800 mm2 [9.0 in2] (i.e., a square 76 mm x 76 mm [3.0 in x

3.0 in]), and 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.

2.1.2 Retained existing coating shall have sufficient

adhesion that it cannot be removed from the substrate by lifting with a dull putty knife. The borders of the retained coating shall have no clear shoulder or edge at the coating/ substrate interface. If the coating can be dislodged, the area shall be rejected and cleaned again until the requirements of this standard are met. Coating that has been tapered by

(1) IEEE/ASTM SI 10, American National Standard for Metric Practice, ASTM International, West Conshohocken, PA, 2017, <https://www.astm.org>

This standard was developed by the SSPC C.2.21 Partial Blast Cleaning Committee and first issued in 2020.

the blasting process in a manner that challenges adhesion and removes all sharp edges is well adhered. Additional information about evaluating the edges of retained coating is contained in Appendix A4.

2.1.3 Retained coating shall have no visible cracks, blisters, delamination, or other defects after the blasting. The retained coating shall not have chalking or residual corrosion staining. The retained coating shall be uniformly roughened and shall not have any area larger than 40 mm2 (~1/16 in)2 that exhibits the appearance of undisturbed coatings.

Additional information about evaluating retained coating is contained in Appendix A4.

2.2 Spot Blasting: 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).

2.3 Sweep Blasting: 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.

Commentary: The techniques referred to as “sweep blasting” and “brush blasting” should not be confused with the surface cleanliness level “brush-off blast cleaning” as defined in SSPC-SP 7/NACE No. 4.

3. Additional Technical Considerations

3.1 Acceptable variations in appearance that do not

affect surface cleanliness as defined in Section 2.1.1 include variations caused by the type of steel, original surface condition, thickness of the steel, weld metal, mill or fabrication marks, heat treating, heat-affected zones, blasting abrasives, and differences resulting from the abrasive blast pattern.

3.2 Appendices A4 and A5 provide additional information

on coating appearance.

3.3 The contractor shall prepare a sample area to serve

as a Job Reference Standard (JRS) for the degree of surface preparation specified. The JRS shall be representative of the surface to be prepared. Following acceptance by the contracting parties, the JRS shall be documented, preserved, or both, to serve as a reference for the duration of the project, and all documentation shall be retained as part of the project records. In any dispute, the written definition set forth in this standard shall take precedence over the JRS or other visual comparators.

3.4 This standard shall be used only when the coating

to be applied on the prepared surface is determined to be compatible with the existing coating.

3.5 Appendix A6 provides guidance on the removal of

pack rust and rust scale prior to blast cleaning. Appendix A7 provides examples of specification statements.

3.6 Coating thickness will vary among surfaces with

retained coating and, by definition, there will be no coating at all on surfaces blasted to near-white metal cleanliness (SSPC-SP 10/NACE No. 2). Appendix A8 provides additional information on final applied coating thickness.

4. Referenced Documents

4.1 The latest issue, revision, or amendment of the

standards listed in Sections 4.3 through 4.6 shall govern unless otherwise specified. Standards marked with an asterisk (*) are referenced only in the Appendices, which are not requirements of this standard.

4.2 If there is a conflict between the requirements of any

of the documents listed in Sections 4.3 through 4.6 and this standard, the requirements of this standard shall prevail.

4.3 SSPC and Joint Standards

* SSPC Guide

Guide for Illumination of Industrial Painting Projects

SSPC Guide

Field Methods for Retrieval and Analysis of Soluble Salts on Steel and Other Nonporous Substrates

SSPC-SP 1 Solvent Cleaning

SSPC-SP 10/

NACE No. 2

Near-White Metal Blast Cleaning

SSPC-SP 10

(WAB)/NACE

WAB-2

Near-White Metal Wet Abrasive Blast Cleaning

SSPC-AB 1 Mineral and Slag Abrasives SSPC-AB 2 Cleanliness of Recycled Ferrous

Metallic Abrasives SSPC-AB 3 Ferrous Metallic Abrasive SSPC-AB 4 Recyclable Encapsulated Abrasive

Media

* SSPC-PA 17 Procedure for Determining

Conformance to Steel Profile/Surface Roughness/ Peak Count Requirements

* SSPC-PA 2 Determining Conformance to Dry Coating Thickness Requirements

* SSPC-SP

COM

Surface Preparation Commentary for Steel Substrates

SSPC-SP

WJ-2/

NACE WJ-2

Waterjet Cleaning of Metals – Very Thorough Cleaning

4.4 ASTM International Standards(2)

* ASTM D3359 Standard Test Methods for Rating Adhesion by Tape Test

(2) ASTM International, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, phone int+1-610-832-9500. For referenced ASTM standards, visit the ASTM website <http://www.astm.org>

ASTM D4285 Standard Test Method for Indicating Oil or Water in Compressed Air

ASTM D4414 Standard Practice for

Measurement of Wet Film Thickness by Notch Gages

ASTM D4417 Standard Test Methods for Field

Measurement of Surface Profile of Blast Cleaned Steel

* ASTM D6677 Standard Test Method for Evaluating Adhesion by Knife

ASTM F21 Standard Test Method for

Hydrophobic Surface Films by the Atomizer Test

ASTM F22 Standard Test Method for

Hydrophobic Surface Films by the Water-Break Test

4.5 International Organization for Standardization

(ISO)(3) Standard:

* ISO 8502-3 “Preparation of steel substrates before application of paints and related products—Tests for the assessment of surface cleanliness— Part 3: Assessment of dust on steel surfaces prepared for painting (pressure-sensitive tape method)”

4.6 NACE International(4) Standards:

* NACE 6G186 Surface Preparation of Soluble Salt Contaminated Steel Substrates Prior to Coating

* NACE SP0178 Design, Fabrication and Surface Finish Practices for Tanks and Vessels to be Lined for Immersion Service

5. Procedures Before Thorough Spot and Sweep Blast Cleaning

5.1 Precleaning: Visible deposits of oil and grease, and heavy deposits of foreign matter such as mud, bird droppings, and caked salts shall be removed by methods in accordance with SSPC-SP 1, or as specified. Appendix A1 provides information about nonvisible contaminants.

5.2 Before beginning cleaning, surface imperfections

such as sharp edges, weld spatter, or burning slag shall be removed from the surface to the extent required by the

(3) International Organization for Standardization (ISO), Case Postale 56, Geneva CH-1211, Switzerland. In the United States, ISO standards may be obtained from the American National Standards Institute (ANSI) at <http://www.ansi.org>

(4) NACE International, 15835 Park Ten Place, Houston,Texas 77084, USA, Phone:

+1-281-228-6200 procurement documents (project specification). Appendix A9 provides additional information.

5.3 The condition of the coated steel prior to blast

cleaning should be documented before the blast cleaning commences.

6. Thorough Spot and Sweep Blast Cleaning Methods and Operation

6.1 Any of the following methods of surface preparation

can be used to achieve a Thorough Spot and Sweep Blast Cleaned surface.

(1) Dry abrasive blasting using compressed air, blast nozzles, and abrasive.

(2) Dry abrasive blasting using a closed-cycle, recirculating abrasive system with compressed air, blast nozzle, and abrasive, with or without vacuum for dust and abrasive recovery.

(3) Dry abrasive blasting using mobile, closed-cycle, recirculating abrasive systems with centrifugal wheels and abrasive.

Hazardous materials may be present. Appendix A10 provides additional information.

6.2 Clean, dry compressed air shall be used for nozzle

blasting. Cleanliness of the compressed air shall be verified in accordance with the procedure described in ASTM D4285.

Moisture separators, oil separators, traps, or other equipment may be necessary to achieve this requirement.

6.3 Wet abrasive blast cleaning (WAB) or pressurized

water cleaning methods may be substituted for dry abrasive blast cleaning if specified in the contract documents. If WAB cleaning is used, the bare steel shall meet requirements of SSPC-SP 10 (WAB)/ NACE WAB-2. If pressurized water cleaning is used, the bare steel shall meet requirements of

SSPC-SP WJ-2/NACE WJ-2.

6.3.1 Information on the use of inhibitors to prevent

the formation of rust immediately after cleaning with water methods is contained in Appendix A11.

6.3.2 Surfaces prepared using wet methods should be

completely dry prior to coating application.

7. Requirements for Blast Cleaning Abrasive Media

7.1 Selection of abrasive size and type shall be based

on the type, grade, and surface condition of the steel to be cleaned, the type of blast cleaning system used, the finished surface to be produced (cleanliness and surface profile [roughness]), and whether the abrasive will be recycled.

7.2 The cleanliness and size of recycled abrasives

shall be maintained to ensure compliance with this standard.

7.3 The blast cleaning abrasive shall be dry and free

of oil, grease, and other contaminants as determined by the test methods found in SSPC-AB 1, SSPC-AB 2, SSPC-AB 3 and SSPC-AB 4, as applicable, or according to requirements of the procurement documents (project specification).

7.4 The abrasive shall comply with any additional

specified requirements or limitations. Additional information on abrasive selection is in Appendix A12.

7.5 The abrasive selected shall roughen the cleaned

surface to produce the surface profile specified in the procurement documents (project specification). If the surface profile is not specified in the procurement documents, the abrasive selected shall roughen the cleaned surface to the degree required by the product data sheet for the coating to be applied. Additional information on surface profile and the film thickness of coating applied over the surface profile is in Appendices A8 and A13.

8. Procedures Following Thorough Spot and Sweep Blast Cleaning and Immediately Prior to Coating

8.1 Visible deposits of oil, grease, and other

contaminants shall be removed by methods in accordance with SSPC-SP 1 or as specified.

8.2 Dust and loose residues shall be removed from

blast cleaned surfaces and retained coating by brushing;

blowing off with clean, dry compressed air; vacuum cleaning;

or other methods specified in the procurement documents (project specification).

8.2.1 The presence of toxic metals in the abrasives

or coating being removed may place restrictions on the methods of cleaning permitted. The chosen method shall comply with all applicable regulations.

8.2.2 Cleanliness of the compressed air shall be verified

in accordance with the procedure described in ASTM D4285.

8.3 After blast cleaning, any surface imperfections

(e.g., sharp edges, weld spatter, burning slag, scabs, slivers) shall be removed to the extent required in the procurement documents (project specification). After removal of surface imperfections, the surface shall be reprofiled by blast cleaning or other methods specified in the procurement documents (project specification). Additional information on surface imperfections is provided in Appendix A9.

8.4 Immediately prior to coating application, the

entire surface to be coated shall comply with the degree of cleanliness defined by this standard. Any visible rust that forms on the surface of the steel after blast cleaning shall be removed by blast cleaning or other methods specified in the procurement documents (project specification) before coating. Information on the effect of dew point (surface condensation) is provided in Appendix A14.

Appendix A. Explanatory Notes (Nonmandatory)

This appendix is considered nonmandatory, although it may contain mandatory language. It is intended only to provide supplementary information or guidance. The user of this standard is not required to follow, but may choose to follow, any or all of the provisions herein.

A1 NONVISIBLE CONTAMINATION: Nonvisible contamination is the presence of organic matter, such as thin films of oil and grease; and inorganic or soluble ionic materials such as chlorides, ferrous salts, nitrates, and sulfates that may be present on the substrate or retained coating.

A1.1 Steel contaminated with water-soluble salts (e.g., sodium chloride, potassium sulfate) may rapidly develop rust-back. Rust-back can be minimized by removing these salts from the steel surface and eliminating sources of recontamination during and after cleaning. Water soluble salts which remain embedded in retained coating may cause osmotic blistering of the new coating system. These contaminants, along with their concentrations, may be identified using the laboratory and field tests described in SSPC-Guide 15. Information regarding methods commonly used in removal of these contaminants can be found in NACE Publication 6G186.

A1.2 Other nonvisible contaminants (e.g., oil, acid, base, silicone, wax) may have an effect on coating performance. Coating manufacturers should be consulted for recommendations of maximum surface contamination allowed. A visual water break test (ASTM F21 or F22) on the surface may be used to detect hydrophobic contaminants.

Ultraviolet (“black”) light can also be used to detect films of hydrocarbons on the surface. Appropriate eye protection should be worn when using ultraviolet light to inspect surfaces.

A1.3 The test method or procedure to be used for determining the level of remaining nonvisible contaminants should be addressed in the procurement documents (project specification).

A1.4 The level of nonvisible contaminants found in an extraction from the surface that may remain on the surface is usually expressed as mass per unit area (µg/cm2 or mg/m2) or as conductivity of a given test solution (µS/cm).

A1.4.1 The following is an example specification for salt contamination based on concentration measurements:

“Immediately prior to the application of the coating, the surface extract shall not contain more than [specify] µg/cm2 [specify contaminant e.g., chloride, total salt] when tested with a field test as described in SSPC-Guide 15 [or identify a specific method].”

A1.4.2 The following is an example specification for salt contamination based on conductivity measurements:

“Immediately prior to the application of the coating, the conductivity of the surface extract shall not exceed [specify] μS/cm when tested with a field tests as described in SSPC-Guide 15 [or identify a specific method].”

A2 FUNCTION: Thorough Spot and Sweep Blast Cleaning is used when the objective is to remove all failing or poorly adhered coatings, establishing a near-white metal level of cleanliness on all exposed steel, and a thorough, uniformly roughened surface on all retained coatings. The primary functions of blast cleaning before coating are:

(a) To remove material from the surface that can cause early failure of the coating system;

(b) To obtain a suitable surface profile (roughness) to enhance the adhesion of the new coating system;

and

(c) To facilitate a uniform, continuous coating film across the surface being protected, minimizing the extent of borders between newly installed repair coating and the existing retained coating.

A3 EXISTING COATING CONSIDERATIONS: The following should be considered when planning a project using thorough spot and sweep blast cleaning procedures.

A3.1 Thorough spot and sweep blast cleaning is intended to subject the existing coating to significant abrasive impact. Coating that cannot withstand the abrasive blasting process should be removed. For thinner or less durable materials, this may result in removal of most of the existing coating. More retained coating will remain for thicker, more durable materials. The objective of this standard is to leave coating that can withstand the abrasive blasting process, not to retain as much extant coating as possible.

A3.2 During abrasive blast cleaning, coating is removed either by the mechanism of impact or by the mechanism of erosion. The intent of this standard is to remove the coating that can be removed by impact rather than erosion, and leave coating which is sufficiently well adhered to resist impact.

During traditional full removal, a sufficiently adhered coating would require longer dwell times for the blast nozzle to erode off the existing coating (a more time-consuming process).

A3.3 Thorough spot and sweep blast cleaning should only be specified when the existing, aged coating is chemically compatible with the coating to be applied over

it. Examples include replacing epoxy linings or maintaining zinc/epoxy/urethane coating systems.

A3.3 Thorough spot and sweep blast cleaning may not be suitable for coatings that are not adequately challenged by the abrasive blasting process to reveal latent film weaknesses. For example, loosely bonded coatings with high cohesive strength (e.g., glass-reinforced or elastomeric linings) may sufficiently resist abrasive blasting, retaining latent defects. A test area may be prepared to determine if the thorough spot and sweep blast cleaning process is appropriate in such instances.

A4 EVALUATING RETAINED COATING: Inspectors and blast cleaning craftworkers should use the following guidance to evaluate surface with retained coating.

A4.1 Color can be a useful indicator for sufficiency of roughening of retained coatings. For example, removing yellowed/faded material can be a visual cue to blast cleaning craftworkers and inspectors that the existing coating was sufficiently blast cleaned. Similarly, removing the contrasting color of a finish coat is also a visual cue suggesting sufficient blast cleaning effort.

A4.2 The gloss or reflectance of adherent coating can be useful in determining if a coating was adequately challenged or uniformly roughened. In general, any retained coating should have no visible retained gloss or reflective appearance after blast cleaning.

A4.3 Retained coating should exhibit a reasonably uniform appearance. The abrasive nozzle pattern used should not be visually obvious as evidenced by “streaks” across the retained coating.

A.4.4 The specifier may choose to require additional cleanliness or roughness tests specifically for the retained coating. Traditional surface preparation inspection tests for dust (ISO 8502-3), and surface profile (ASTM D4417) have been used, though the standards are not written for polymer surfaces. For example, the surface profile of retained paint may be considerably deeper than profiled steel, which may limit the ability to use some profile measurement techniques.

Visual inspections for embedded grit in the retained coating might also be considered, though there is not a current standard test method. Specific tests for adhesion of the retained coating such as ASTM D6677 or ASTM D3359 may also be specified.

A4.5 Visual inspections should be aided by a minimum of 540 lux (~50 foot-candles) illumination per SSPC-Guide 12 when inspecting a surface following blast cleaning to

SSPC-SP 18.

A5 PREPARING AND EVALUATING EDGES OF

RETAINED COATING: The intent of this standard is to remove as much of the underlying coating as is practical without the excessive effort that may be required for complete removal of tightly adherent coating. The edges of retained coating pose a performance risk if not properly prepared. The edges of retained coating meeting the requirements of Section 2.1 will generally exhibit a mottled, tapered appearance. While the abrasive blasted edges may be described as “feathered,” feathering using hand or power tools is neither beneficial nor desired.

A6 PACK RUST AND RUST SCALE: It may be helpful to use hand or power tools to remove heavy deposits of pack rust and rust scale before beginning the blast cleaning process.

A7 SPECIFICATION STATEMENTS:

A7.1 The following are examples of specification statements:

“All surfaces to be recoated shall be cleaned in accordance with the procedures in SSPC-SP 18 and shall meet the requirements of SSPC-SP 18 immediately prior to coating application.”

“All surfaces to be recoated shall be cleaned in accordance with the procedures in SSPC-SP 18 and shall meet the requirements of SSPC-SP 18 immediately prior to coating application. Tightly adherent coating shall not remain on more than [specify percentage] of the total area with no single area of retained coating greater than [specify size of the area] square feet.”

A7.2 The specifier should consider whether a surface should be cleaned as required to achieve a particular, not to exceed maximum, level of nonvisible contamination prior to recoating. Suggested specification statements for nonvisible contamination are given in Appendices A1.4.1 and A1.4.2.

A8 FILM THICKNESS: Coating thickness is critical to adequate performance. At a minimum, the dry film thickness of the entire coated area should meet the coating manufacturer’s recommended thickness, though areas of retained coating may have a higher allowed thickness. To ensure that coating thicknesses are properly measured, the procedures in SSPC-PA 2 for verification of accuracy of Type 1 and Type 2 gages should be used.

A8.1 It is essential that ample coating be applied after blast cleaning to adequately cover the peaks of the surface profile of exposed steel. The dry film thickness of the coating above the peaks of the surface profile should be adequate for the desired protection. If the dry film thickness over the peaks is inadequate, premature rust-through or coating failure will occur.

A8.2 Sufficient coating should be applied over the retained coating to (a) provide a uniform, continuous film over the retained coating and bare metal and (b) meet the overall coating thickness requirements for the specified system. The presence of an existing coating should not in any circumstance allow the completed coating system to be less than the specified thickness. Since the thickness of the new and retained coating could be considerably higher than the required coating thickness, the specifier should consider requiring that coating thickness be determined according to SSPC PA 2, Coating Restriction Level 5.

A8.3 It is beneficial to have alternative colors or other visual distinction between the retained and newly applied coating.

A8.4 A uniform, continuous film over retained coating will not exhibit the roughness of the originally prepared coating and should have sufficient hiding power to cover the aged coating.

A8.5 In-process wet film thickness measurements should be measured by the coating applicator per ASTM D4414 in order to ensure adequate film coverage. This can be especially useful for ensuring proper coverage over areas of retained coating.

A9 SURFACE IMPERFECTIONS:

A9.1 Surface imperfections that can cause premature coating failure are often present. Coatings tend to pull away from sharp edges and projections, leaving little or no coating to protect the underlying steel. Other features that are difficult to properly cover and protect include crevices, weld porosities, and laminations.

A9.2 Poorly adhering fabrication defects, such as weld slag residues, loose weld spatter, and surface laminations may be removed during the blast cleaning operation. Other surface defects such as steel slivers, weld porosities, or deep corrosion pits may not be evident until the surface preparation has been completed. Therefore, proper planning for such surface repair work should be given prior consideration, because the timing of the repairs may occur before, during, or after the blast cleaning operation. The SSPC-SP COM and NACE SP0178 contain additional information on surface imperfections.

A9.3 The high cost of the methods to remedy surface imperfections (e.g., edge rounding and weld spatter removal) should be compared with the benefits of preventing premature coating failure. Therefore, those responsible for establishing the requirements and those responsible for performing the work should agree on the procedures to be used to repair surface imperfections to the extent required in the procurement documents (project specification).

A10 HAZARDOUS MATERIAL: The presence of hazardous material in the coatings, cleaning media, or in the work area itself can place restrictions on the methods of cleaning permitted. Dry abrasive blast cleaning is often used to remove coatings with hazardous components.

Applicable industrial hygiene tests should be performed, and appropriate industrial hygiene practices should be followed.

A11 USE OF CORROSION INHIBITORS: It may be advantageous to add corrosion inhibitors to the surface preparation water or apply them to the surface immediately after cleaning by wet methods in order to temporarily prevent rust formation. Some corrosion inhibitor treatments may interfere with the performance of certain coating systems.

The coating manufacturer should be consulted to ensure the compatibility of corrosion inhibitors with the coatings.

A12 ABRASIVE SELECTION: Types of metallic and nonmetallic abrasives are discussed in SSPC-SP COM.

Blasting abrasives may become embedded in, or leave residues on, the surface of the steel and retained coatings during cleaning. While such embedment or residues are normally not detrimental, care should be taken to ensure that the abrasive is free from detrimental amounts of water-soluble, solvent-soluble, acid-soluble, or other soluble contaminants (particularly if the cleaned steel is to be used in an immersion environment). Criteria for selecting and evaluating abrasives are found in SSPC-AB 1, SSPC-AB 2, SSPC-AB 3, and SSPC-AB 4.

A13 SURFACE PROFILE:

A13.1 Surface profile is the roughness of the surface that results from abrasive blast cleaning. Profile height and uniformity is dependent on the size, shape, type, and hardness of the abrasive, particle velocity and angle of impact, hardness of the surface, amount of abrasive recycling, and the proper maintenance of working mixtures of grit and shot.

A13.2 The allowable minimum/maximum height of profile is usually dependent on the thickness of the coating to be applied. Large abrasive particles (particularly if metallic) can produce a surface profile that may be too high to be adequately covered by a single thin-film coat.

Accordingly, the use of larger abrasives should be avoided in these cases. However, larger abrasives may be needed for thick-film coatings or to facilitate removal of thick coatings, heavy mill scale, or rust. If control of surface profile (minimum/maximum) is deemed to be significant to coating performance, it should be addressed in the procurement documents (project specification). Typical surface profile heights achieved with commercial abrasive media are shown in Table 4 of SSPC-SP COM. The surface profile should be measured in accordance with ASTM D4417 or as specified.

Compliance with specified profile requirements should be determined in accordance with SSPC-PA 17.

A13.3 While it is possible to measure the surface profile of roughened coatings, for most purposes it is not necessary.

A visual assessment of the surface roughness uniformity is adequate. If traditional techniques are employed to measure the roughness of the retained coating, profiles will generally be deeper and rougher than are commonly achieved with steel. Specification limits may also vary with the existing coating type.

A14 DEW POINT: Moisture condenses on any surface that is colder than the dew point of the surrounding air.

Therefore, the temperature of the steel surface should be at least 3 °C (5 °F) above the dew point during dry blast cleaning and coating operations. It is advisable to visually inspect for moisture and periodically check the surface temperature and dew point during blast cleaning operations and to avoid the application of coating over a damp surface.

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