Often, the ASTM standards for the hot-dip galvanizing industry cite SSPC surface preparation standards. What are these and which ones are important to the industry?

The Society for Protective Coatings (SSPC) publishes and maintains many standards relevant to the coating industry. Standardized subjects include requirements for paints, standards for evaluating the qualifications of contractors, surface preparation standards, and other topics. The ASTM specifications relevant to the hot-dip galvanizing industry often reference these surface preparation standards. Here is a list and brief summary of the surface preparation standards used in the hot-dip galvanizing industry with a brief summary of each.

SSPC-SP1 Solvent Cleaning

The method of solvent cleaning is intended to remove all visible oil, grease, soil, drawing and cutting compounds, and all other soluble contaminants from steel surfaces. The specification calls for the removal of dust before solvent cleaning by the use of a stiff bristle brush. After solvent cleaning, dust is to be removed by either blowing off with clean dry air or vacuum cleaning.  The specification lists several different ways in which solvent cleaner may be applied to the surface including wiping or scrubbing the surface with solvent, spraying solvent directly onto the surface, vapor degreasing, immersion, alkaline cleaners, and steam cleaning.

The preparation of a hot-dip galvanized coating for paint or powder coating connects this standard to the galvanizing industry. ASTM D6386 and D7803 both call out this standard during their surface preparation steps. Solvent cleaning is used to  remove oil and grease from the galvanized coating before it is profiled and cleaned with other methods.

SSPC-SP2 Hand Tool Cleaning

This standard covers the removal of loose mill scale, loose rust, loose paint, and other loose detrimental foreign matter from surfaces with the use of non-powered hand tools. Adherent products as determined by a dull putty knife are not to be removed with this procedure. SSPC-SP1 removes oil and grease before cleaning with hand tools, and compressed air or a vacuum will remove dust after this cleaning procedure is completed. Removing all weld slag and stratified rust with impact tools, using non-impact methods such as scraping to remove all non-adherent materials, and feathering an adherent paint coating, are all hand tool cleaning methods accepted by this standard.

The preparation for painting and powder coating over a hot-dip galvanized coating, as well as the repair of damaged and uncoated areas, use SSPC-SP2 as a standard surface preparation procedure. The specifications for preparing hot-dip galvanized steel for paint and powder coating ASTM D6386 and D7803 respectively list hand tool cleaning as an acceptable method for smoothing the surface and removing zinc high spots of newly galvanized and partially weathered coatings. Hand tool cleaning also acceptably cleans a hot-dip galvanized surface before surface profiling according to D6386. ASTM A780 also uses SSPC-SP2 as an acceptable preparation method for repair by the use of paints containing zinc dust.

SSPC-SP 3 Power Tool Cleaning

SSPC-SP3 details the use of power assisted tools for surface preparation. The preparation method described here is intended to remove the same contaminants from a surface as in SSPC-SP2, and the procedures before and after using this cleaning method are identical to SSPC-SP2. The standard includes using rotary, impact, or power brushing tools to remove stratified rust, weld slag, and mill scale. Power abrading tools also acceptably remove mill scale according to the standard. SSPC-SP3 requires power tools to be used in a manner that prevents the formation of burrs, sharp ridges, and sharp cuts. Feathering the edges of adhered paint surfaces is also required.

Just as SSPC-SP2 and SSPC-SP3 have similar requirements for surface preparation, so too are they similar in their applications to the hot-dip galvanizing industry. In preparation for paint and powder coating, both standards are mentioned by D6386 and D7803 in the same section. They are considered interchangeable in their use of smoothing the coating and removing zinc high spots before the zinc coating is further prepared. D6386 also considers the two methods interchangeable during the surface cleaning phase of the process. A780, however, does not specifically call out SSPC-SP3 for the preparation of a damaged coating for repair by zinc rich paints.

SSPC-SP5 White Metal Blast Clean

A white metal blast will remove all coatings, mill scale, rust, oxides, corrosion products, and all other foreign products from a steel surface. SSPC-SP5 provides conditions for the end condition of a white metal blast, and the materials and procedure necessary for providing the end condition. The standard lists dry abrasive blasting as the preferred method with wet blasting an option to be agreed upon between the two parties. The size and type of abrasive media to be used depends on the profile specified by the coating system to be used. The standard then requires oil, grease, and dust to be removed from the surface and the work inspected.

SSPC-SP5 is not specified often in standards for the hot-dip galvanizing industry. However, A780 uses the white metal blast as the surface preparation standard for repairing a zinc coating using sprayed zinc.

SSPC-SP7 Brush Off Blast Clean

This standard states a brush of blast clean will allow as much of an existing adherent coating to remain as possible and to roughen its surface prior to coating application. This blasting method cleans less aggressively than an industrial blast clean. A provided definition of a brush-off blast cleaned surface describes a surface free of all visible oil, grease, dirt, dust, loose mill scale, loose rust, and loose coating when viewed without magnification. The surface may still include profiled and tightly adherent mill scale, rust, and coating.

Hot-dip galvanized coatings contain a layer of zinc oxide and zinc hydroxide which would be detrimental to the adherence of a topcoat. A paint coating also requires a profile typically not found on the relatively smooth surface of a hot-dip galvanized coating. D6386 lists sweep blasting as an acceptable surface preparation method with its own given requirements for the procedure but calls out SSPC-SP7 for more procedure specifications.

SSPC-SP10 Near-White Metal Blast Cleaning

Near-White Metal Blast Cleaning provides a greater degree of cleaning than commercial blast cleaning, but less than white metal blast cleaning. The SSPC-SP10s definition of a near-white metal blast clean requires the surface to be free of all visible oil, grease, dust, dirt, mill scale, rust, coating, oxides, corrosion products, and other foreign matter when viewed without magnification. The standard also provides requirements for random staining on the surface which shall not exceed five percent of each unit area of surface. The surface shall be cleaned of oil, grease, and dust after the near-white metal blasting procedure.

ASTM A780 calls for a near-white metal blast cleaning when field conditions include immersion. In this case the area damaged and in need of repair by a paint containing zinc dust will receive a blast per SSPC-SP10 before the paint is applied.

SSPC-SP11 Power-Tool Cleaning to Bare Metal

This surface preparation standard is similar to that some less aggressive blast cleans. The standard requires the surface to be free from all visible oil, grease, dirt, dust, rust, coating, oxides, mill scale, corrosion products, and other foreign matter when viewed without magnification. However, some rust remaining at the bottom of pits is acceptable if the surface was pitted to begin with. A surface profile of no less than one mill between peaks and valleys is required. Grinding tools and impact tools are both acceptable means for achieving the level of cleaning specified. Oil, grease, and dust must be removed both before and after the cleaning procedure.

The power-tool cleaning method of SSPC-SP11 will provide an adequate profile for a paint system to adhere to. This fact makes power-tool cleaning an acceptable method for surface preparation of a hot-dip galvanized coating after it has been cleaned. D6386 states that painting should be performed as soon as possible after power-tool cleaning or surface grinding. A780 allows uncoated areas to be power-tool cleaned before repair with zinc rich paints in areas of less critical field exposure conditions.

SSPC-SP12 Surface Preparation and Cleaning of Metals by Waterjetting Prior to Recoating

Waterjetting is the use of standard jetting from a nozzle at high pressures (10,000 psi or higher) to prepare a surface for coating. Intended primarily for carbon steel, but applicable on other metals, waterjetting does not create a profile on the metal substrate. However, metals with an existing profile acceptable for painting can be cleaned with waterjetting to expose the existing profile. Waterjetting will remove all loose rust, loose mill scale, and loose coatings uniformly while meeting the conditions of WJ-1, WJ-2, WJ-3, and WJ-4.

A hot dip-galvanized coating is said to be fully weathered when the exposed surface consists mainly of water-insoluble zinc carbonates, some zinc oxides, and rarely, zinc hydroxides. Usually occurring around after six months of weathering, a finely etched and roughened profile acceptable for paint covers the surface of the galvanized coating. After all oil and grease has been removed with the methods of another surface preparation standards, cleaning with a waterjet according to SSPC-SP12 will remove all lose particles and leave the finely etched surface previously described. D7803 specifically calls out this standard with a condition that the water pressure be below 1450 psi. D6386 describes the same process, but without mentioning the surface preparation standard.

SSPC-SP16 Brush-Off Blast Cleaning of Coated and Uncoated Galvanized Steel, Stainless Steels, and Non-Ferrous Metals

Use of this surface preparation standard roughens and cleans the bare substrate including a hot-dip galvanized coating to create a profile suitable for painting. Completion of this process produces a profiled surface free of all visible oil, grease, dirt, dust, metal oxides (corrosion products), and other foreign matter when viewed without magnification. The standard also contains unique procedures for use on galvanized steel including inspection for a passivation treatment and wet storage stain.

The brush off-blast cleaning procedure described in SSPC-SP16 provides an acceptable profile on hot-dip galvanized surfaces for the application of paint or powder coat. After a galvanized surface has been acceptably cleaned of oil, grease, and similar products, this brush blasting can be applied in order to create an acceptable profile. D7803 uses this standard for the preparation of a galvanized coating for powder coating. D6386 does not specifically call out this standard, but the procedure is very similar to that of D7803.

Below is a table of each SSPC spec and where it relates with galvanizing ASTM specs: