Steel

Standard Guide for Painting Inspectors (Metal Substrates) – ASTM D3276

Surface Preparation Methods and Requirements

Pictorial Standard D2200 (SSPC-VIS 1) should be provided to the inspector on a job involving blast cleaning of structural steel.

Factors Affecting Coating Performance

Cleanliness
Many materials, if not removed from the surface, will affect the life of the coating. These include oil, grease, soil, weld spatter, and slag, that make it impossible to obtain proper adhesion to the metal surface. Surface soluble salts shall be removed to the degree specified or long-term coating performance may be adversely affected. SSPC/NACE issues detailed surface preparation specifications that cover various methods for cleaning.

Mill Scale
The bluish-black oxide resulting from the hot-rolling process, is a constant source of trouble leading to coating failure. This scale is very brittle and can crack or loosen due to temperature changes (both in fabricating and weathering in the field) leading to failure of the coating.

Surface Profile
The texture of the metal surface has a significant effect on the performance of coatings, since it increases the surface area to which the coating can develop adhesion. In fact, the term “anchor pattern” is sometimes used to describe the depth of profile.

Sharp Edges
Sharp edges and rough welds can compromise coating performance, particularly in immersion service. Sharp edges and outside corners should be radiused, and rough welds should be smoothed or ground flat. NACE SP0178 and SSPC-PA Guide 11 may be used as guides.

On complex structures, all dirt and debris should be removed from pockets, crevices, obstructed areas such as gusset plates and connections, and tops of horizontal surfaces. Blasting debris that accumulates on horizontal surfaces should be removed. Special attention should be placed on examining hard to reach areas, the back side of nuts and bolts, sides of members in close proximity to other members or walls, and undersides of members.

Appendix
(Nonmandatory Information)
X1. Inspection Checklist

Surface Preparation Commentary for Metal Substrates – SSPC-SP COM

Table 1A
Summary of Current SSPC Abrasive and Surface Preparation Standards

Surface Conditions
New Construction: For new construction there are four surface conditions based upon the rust condition classifications. These initial conditions, defined in SSPC visual consensus guides, namely, SSPC-VIS 1, SSPC-VIS 3, and SSPC-VIS 4, are:
Condition A: Steel surface covered completely with adherent mill scale; little or no rust visible
Condition B: Steel surface covered with both mill scale and rust
Condition C: Steel surface completely covered with rust; little or no pitting visible
Condition D: Steel surface completely covered with rust; pitting visible
Conditions A, B, C, and D are also referred to as Rust Grades A, B, C, and D.

Surface Contaminants
Typical contaminants that should be removed during surface preparation are rust, corrosion products, mill scale, grease, oil, dirt, dust, moisture, soluble salts containing anions, such as chlorides, sulfates (if present in detrimental quantities), paint chalk, and loose, cracked, or peeling coating.

Mill Scale
Mill scale is a bluish, somewhat shiny oxide residue that forms on steel surfaces during hot rolling. Although initially tightly adherent, it eventually cracks, pops, and disbonds. Unless completely removed before coating, mill scale will eventually cause the coatings to crack and expose the underlying steel. Steel is anodic to mill scale and so corrodes more rapidly in this combination of “dissimilar metals.”
Mill scale varies in its effects upon the performance of coatings. Tightly adhered or intact mill scale may not need to be removed for mild atmospheric service. If, however, the steel surface will be coated with primers with low wetting properties or exposed to severe environments, such as chemical exposures or immersion in fresh or salt water, then removal of mill scale by blast cleaning or power tool cleaning is necessary. Note that the effort required to remove all tightly adherent mill scale usually results in a surface that has less staining than the maximum 33% permitted by SP 6 or SP 15, but may have more staining than the maximum 5% permitted by SP 10

Surface Defects
Coatings tend to draw thin and pull away from sharp edges and projections, leaving little or no coating to protect the underlying steel, thereby increasing the potential for premature coating failure. This results in poor coating film quality. Other features of steel that are difficult to properly cover and protect include crevices, weld porosity, laminations, etc. These considerations are discussed below. The high cost to remedy these surface imperfections requires weighing the benefits of remedial methods, such as edge rounding or grinding, versus a potential coating failure.
Poorly adhering contaminants, such as weld slag residues, loose weld spatter, and some minor surface laminations, may be removed by abrasive blast cleaning. Other surface defects, such as steel laminations, weld porosities, or deep corrosion pits, may not be evident until after abrasive blast cleaning is completed. Therefore, the timing of such surface repair work may occur before, during, or after preliminary surface preparation operations have begun.

Welds and Weld Spatter
Weld spatter should be removed prior to blast cleaning. Most weld spatter, except that which is very tightly adherent, can be readily removed using a chipping hammer, spud bar, or scraper. Tightly adhering weld spatter may require removal by grinding. Weld spatter that is not removed will result in a lower coating film thickness (as on sharp edges) and may disbond from the base metal, resulting in adhesion failure. Welds can also have sharp projections that may penetrate through the wet coating film. NACE SP0178, “Standard Recommended Practice, Fabrication Details, Surface Finish Requirements, and Proper Design Considerations for Tanks and Vessels to Be Lined for Immersion Service,” provides details on grinding welds.

Sharp Edges
Sharp edges, such as those normally occurring on rolled structural members or plates, as well as those resulting from flame cutting, welding, grinding, etc., and especially shearing, have an influence on coating performance and may need to be removed (e.g., grinding, mechanical sanding, filing). During removal operations, care should be taken use proper grinding techniques to prevent the formation of new sharp edges. SSPC-PA Guide 11, “Guide to Methods for Protection of Edges, Crevices, and Irregular Steel Surfaces.” provides additional information to assist specifiers in determining how to treat edges. Some high solids coatings, often requiring plural component spray, have edge-retentive properties that may lessen the effect of sharp edges.

Table 4 Approximate Profile Height of Blasted Steel

These profile heights are typical if the nozzle pressure is between 90 and 100 psi

Shop, Field, and Maintenance Coating of Metals – SSPC-PA 1

General Requirements for Application of Coatings

Coating shall be applied in accordance with the manufacturer’s written instructions and the project specification. If there is a conflict between the requirements of the manufacturer and the project specification, the more restrictive requirements shall be used unless an exception is approved by the facility owner or authorized representative prior to beginning the application process.

Number Of Coats And Type Of Coating
The number of coats, type of coating, and surfaces to be coated are normally specified in the procurement/specification documents. If coating thickness is not specified, the dry film thickness (DFT) shall comply with the coating manufacturer’s written recommendations. The coating application shall be scheduled to provide protection to the substrate at all construction stages

Stripe Coat Application
When stripe coating is specified in the procurement/specification documents, all corners, crevices, rivets, nuts, bolts, welds, and sharp edges shall be stripe coated. Unless otherwise specified or approved by the facility owner, the stripe coat shall be applied by brush before applying a full coat. The stripe coat shall extend a minimum of one inch (2 cm) from the edge it is protecting. The coating manufacturer’s recommended recoat windows for the stripe coat and full coat shall be observed

Application Near Areas To Be Welded
If the coating specified can be damaged by the heat of welding, or will create fumes hazardous to the welder when heated during the welding process, or is detrimental to the welding operation or the finished welds, the metallic substrate shall not be coated within four inches (100 millimeters) of the areas to be welded, except when using inorganic zinc-rich primer, which may be applied to within two inches (50 millimeters) of the weld area.

Application Continuity
To the maximum extent practical, each coat shall be applied as a visually continuous film of uniform thickness free of holidays, pinholes, and other defects (including runs, drips, sags, etc.). All thin spots or areas missed in the application shall be recoated and permitted to dry before the next coat of coating is applied.

Dry Film Thickness (DFT)
Unless otherwise specified in the procurement/specification documents, all dry film thickness determinations shall be performed as specified in SSPC-PA 2. If no Coating Thickness Restriction Level is specified, Level 3 shall be the default level.

Recoat Windows
Each coating layer shall be in a proper state of cure or dryness before the application of the succeeding coat so that it is not adversely affected by topcoating. Consult the coating manufacturer for the appropriate time interval before recoating. When applying multiple coats of two-component thermosetting systems, topcoats shall be applied within the recoat window specified by the manufacturer of the undercoat in order to obtain good intercoat adhesion. If, for any reason, this time period is exceeded, the undercoat surface shall be treated as recommended by its manufacturer before topcoating. Such treatments include mild abrasion, solvent treatment, and use of a tie coat. Remove surface contamination that could affect the adhesion of subsequent coats.

Contact Surfaces
Contact surfaces shall be coated or left uncoated as specified in the procurement/ specification documents.

Contact surfaces of members to be joined by high strength bolts in a friction connection (faying surfaces) shall provide the required slip coefficient based on the specified class of slip resistance based on the design criteria.

Inaccessible Areas
Metallic surfaces that will be inaccessible for coating after assembly or field erection shall receive the full specified coating system before assembly or erection.

Inspection Requirements
The contractor’s work plan (or the inspector’s inspection plan) shall include hold points prior to each step of the application as described in the SSPC Guide for Planning Coating Inspection (2008). These include:
Pre-surface preparation
Post-surface preparation
Coating conditions for application
Coating application
Post-application of coating
Post-curing