Tips on Painting Structural Steel

    by Donald K. Turner, P.Eng.

    Editor's note: Don Turner retired from his position as Vice-President of CISC on December 31, 1993. During his long and illustrious career with CISC he was responsible for liaison with the Steel Structures Painting Council and served as Secretary of the CISC committee on Painting Structural Steel.

    1. Painting Of Steel Is Not Always Required

    The inside environment of most buildings to-day is conditioned for human occupancy (low humidity). Detrimental rusting of steel takes place when the relative humidity is above 70%. Therefore, if steel is to be exposed for a short period during construction and subsequently clad and enclosed, the structural steelwork will not corrode and does not need any paint protection. One exception is steel beams for housing. NBCC PART 9 Housing & Small Buildings, clause 9.23.8.2 requires steel beams to be shop primed.

    The interior structural steel of the 72-storey First Canadian Place, Toronto, (Canada's tallest building) has no paint. Specifying no paint saves the costs for cleaning, painting, inspection, touch-up paint, and the cost of the paint materials.

    2. Open Web Steel Joists (OWSJ)

    When this structural steel component is used in buildings with a non-corrosive environment, savings can be achieved by not painting the un-exposed OWSJ, (e.g., when clad with fire protection materials, hidden by a ceiling, etc.). Delivery scheduling is important to avoid extended outside storage of unpainted joists.

    Due to its configuration, when painting is required, the best method of coating a joist is by immersion. Therefore, it is recommended that the paint specified be of the type and colour which is standard with the OWSJ manufacturer (CAN/CSA S16.1 M89 clause 16.10). Special paints, colours, and coatings should be avoided because they have to be applied individually to each joist by spraying or other means. OWSJ are difficult to coat by spraying and the wasted over-spray may become an environmental problem. Storage or disposal of left-over special coatings present similar environmental difficulties and added cost.

    3. Appearance

    In buildings of low humidity and a non-aggressive atmosphere where the steelwork is exposed to view, the purpose of painting is for appearance only (no corrosion). Minimum cleaning and painting (one coat) should be satisfactory for the life of the building (i.e., office, school, hospital, heated warehouse, etc.).

    If the steelwork is to be an architectural feature using colour, compatibility between the prime coat (usually shop applied) and the top coat (field applied) must be considered since the prime paint will be purchased by the steel fabricator (steel section of the project specification) and the field paint by the painting contractor (painting section of the spec.).

    4. Staining Of Adjacent Materials

    During the construction period, certain types of cladding (e.g., stone work) at the perimeter may become stained by run-off of rust if perimeter steel is left unpainted. CISC/CPMA Standard 1-73a, A Quick-drying One-coat Paint For Use On Structural Steel, which includes cleaning requirements, is recommended for the perimeter steel only (CAN/CSA S16.1 M89 clause 29.5). This one-coat paint is not intended as a primer because it is intended for short-term prevention of rusting, and a top coat is not anticipated.

    5. Bonding With Applied Materials

    Steelwork which is to be encased in concrete, or coated with direct applied fire protection materials, need not be painted. However, unless composite structural behaviour between the applied material and the surface of the steel is part of the structural design, it is not necessary to remove the paint if the steel has been painted, unless the type of paint or coating seriously impairs adherence of the applied material.

    6. Protection Against Corrosion

    Corrosion can be considered under two headings: a) due to exposure to weather; and, b) due to aggressive environments (structures enclosing swimming pools; car washes; water, sewage, composting treatment processes; chemicals aggressive to steel, etc).

    6.1 Exposure to Weather

    6.1.1 Location

    The severity of exposure to weather can vary over short distances, sometimes dramatically, due to factors such as, wind, spray, salt fog, condensation, fumes, and the use of de-icing chemicals. For example, if a highway is located in a land depression, salt spray from traffic may accumulate on the flanges of plate girders supporting an overpass. In such locations, box or bathtub girders are recommended for the overpass to eliminate flanges where salt can accumulate.

    6.1.2 Weathering Steel

    Painting can be avoided by using steels conforming to: CAN/CSA G40.21 M Type A - Atmospheric Corrosion Resistant Weldable Steel; or, Type AT - Atmospheric Corrosion Resistant Weldable Notch Tough Steel (for bridges). Weathering Steel has become standard for bridges in most provinces and has performed satisfactorily throughout Canada, many bridges having been in service over twenty-five years. Weathering Steel used on the exterior of buildings has also been successful, especially where careful attention has been paid to detailing joints to avoid wet spots or pockets where water can collect. Noteworthy in this regard is the Stelco Tower, an office building in Hamilton, Ontario.

    6.1.3 Protected Steel

    The life of protected steel depends on the quality of the system chosen and the care taken in its application. A coating system includes surface preparation, prime coat(s) and top coat(s).

    It is recommended that the surface preparation specifications of the Steel Structures Painting Council (SSPC) be used to specify cleaning prior to painting (see Table 1, page 14, and CAN/CSA S16.1 M89 clause 29.3).

    There are four levels of surface preparation by blast cleaning: Brush-off, Commercial, Near White and White (listed in order of increased degree of cleaning).

    Steel work exposed to weather should use a surface preparation level of at least SP.6 Commercial. The paint system selected (prime and top coats) must be compatible with SP.6 (see Table 2, page 15).

    6.2 Aggressive Environments

    Each environment will have an appropriate coating system (see Table 2). It is recommended that a painting consultant and personnel familiar with the maintenance of steelwork in similar aggressive atmospheres be consulted regarding the selection of suitable systems. Surface preparation, humidity and temperature when applying coatings are important.

    Inspection of each step in applying the total system is recommended. Curing times between coats can be significant. Twelve months following completion, examination of the steelwork is recommended. Coating failure, due to errors regarding the selection of the system or the application of the coatings, will often become apparent in the first year of service and this is the appropriate time to carry out a repair.

    Assess coating failures carefully. The volume of rust forming on structural (carbon) steel is approximately sixteen times the volume of the steel sacrificed, i.e., 1 mm of steel produces a 16 mm layer of rust. Therefore assess the thickness of steel remaining not the amount of rust removed.

    Hot dip galvanizing or metallizing with zinc or aluminum should be considered for aggressive environments or where long-term protection is desired. When such surfaces are to be also painted, the Canadian General Standards Board (CGSB) 85-GP-16M 1979 Standard For: Painting Galvanized Steel is recommended reading. Alternatively, the galvanized surface can be roughened to provide a bonding with the paint coat by carefully blasting the surface with grit.

    7. New Coatings Anticipated

    Studies are being conducted in Canada to assess the need for government regulations regarding levels of volatile organic compounds (VOC) contained in coatings. VOC's (solvents) evaporate as coatings are applied, dry and cure, and may be an environmental or health concern.

    New coatings, with low, or no VOC's, such as water borne paints, have been developed for use where low levels of VOC fumes are necessary.

    8. Coating Companies

    In many parts of Canada, companies are available which specialize in surface preparation and applying coatings to structural steel. This work would be sublet by the structural steel contractor following fabrication.

    Facilities are also available for hot dip galvanizing and metallizing using zinc or aluminum. Since the cost of transportation to and from these coating facilities is a consideration, the location of such facilities needs to be determined when selecting a coating system requiring extreme care in its application.

    SUMMARY

     

    1. Don't paint unless necessary - frequently not necessary.

    2. Weathering Steel often eliminates the need for paint.

    3. Galvanizing or metallizing may be a cost-effective long-term solution.

    4. If paint is necessary, specify the most suitable system: cleaning, prime coat, top coat(s).

    SSPC provides guidance.

    5. Inspect each step of application.

    6. Assess coating failures carefully. Measure the steel remaining, not the thickness of rust.


     

    TABLE 1

    SUMMARY OF SURFACE SPECIFICATIONS

    SPECIFICATION AND SUBJECT DESCRIPTION
    SP1
    Solvent Cleaning    		 Removal of oil, grease, dirt, soil, salts, and contaminants by cleaning with solvent, vapour, alkali, emulsion or steam.
    SP2
    Hand Tool Cleaning    		 Removal of loose rust, loose mill scale, and loose paint to degree specified, by hand chipping, scraping, sanding and wire brushing.
    SP3
    Power Tool Cleaning    		 Removal of loose rust, loose mill scale, and loose paint to degree specified, by power tool chipping, descaling, sanding, wire brushing and grinding.
    SP5
    White Metal Blast Cleaning    		 Removal of all visible rust, mill scale, paint and foreign matter by blast cleaning by wheel or nozzle (dry or wet) using sand, grit or shot. (For very corrosive atmosphere where high cost of cleaning is warranted.)
    SP6
    Commercial Blast Cleaning    		 Blast cleaning until at least two thirds of each element of surface area is free of all visible residues. (For rather severe conditions of exposure.)
    SP7
    Brush-Off Blast Cleaning    		 Blast cleaning of all except tightly adhering residues of mill scale, rust and coatings, exposing numerous evenly distributed flecks of underlying metal.
    SP8
    Pickling    		 Complete removal of rust and mill scale by acid pickling, duplex pickling or electrolytic pickling.
    SP 10
    Near-White Blast Cleaning    		 Blast cleaning nearly to White Metal cleanliness, until at least 95% of each element of surface area is free of all visible residues. (For high humidity, chemical atmosphere, marine or other corrosive environment.)
    SP 11
    Power Tool Cleaning to Bare Metal     		Complete removal of all rust, scale and paint by power tools, with resultant surface profile.


     

    TABLE 2

    TYPICAL ZONE* DEFENCE PAINTING

    ENVIRONMENT SYSTEM

    Zone

    0

    		 Dry interior where structural steel is imbedded in concrete, encased in masonry, or protected by membrane or non-corrosive contact type fireproofing. Leave unpainted.

    Zone

    1A

    		 Enclosed interior, normally dry (or temporary protection).

    Very mild. (Oil base paints would last 10 years or more.)

    		 1. Leave unpainted.

    2. CISC/CPMA 1-73a over nominal cleaning.

    3. CISC/CPMA 2-75 over Brush Off Blast Cleaning if top coating specified.
    Zone

    1B

    		 Exteriors normally dry. (Includes most areas where oil base paints now last 6 years or more and where bare steel corrosion rate is moderate.) 1. CISC/CPMA 2-75 over Brush Off Blast Cleaning, one field coat enamel.

    2. CISC/CPMA 2-75 over Commercial Blast Cleaning, if longer durability required, one or two field coats enamel. 37-50 micrometres (1-2 mils) dry film thickness per coat, minimum system dry film thickness, 75-100 micrometres (3-4 mils).
    Zone

    2A

    		 Frequently wet by fresh water. Involves condensation, splash, spray, or frequent immersion.

    (Oil base paints now last 5 years or less.)

    		 Inorganic zinc primer over Commercial Blast Cleaning 63-75 micrometres (2-3 mils dry). Field coat of 75-100 micrometres (3-4 mils) high build, vinyl or epoxy.
    Zone

    2B

    		 Frequently wet by salt water. Involves condensation, splash, spray or frequent immersion.

    (Oil base paints now last 3 years or less.)

    		 1. Inorganic Zinc Primer over Near-White Blast Cleaning 63-75 micrometres (2-3 mils) dry. Field coat 75-100 micrometres (3-4 mils) high build vinyl or epoxy.

    2. Same as 1 above, but additional top coat 75-100 micrometres (3-4 mils) dry. Assure satisfactory adhesion of top coats.
    Zone

    3

    		 Chemical exposures (acidic, alkaline, oxidizing, solvents etc.). Same as Zone 2B but with chemically resistant finish coat system specially chosen to protect primer and base metal against specific chemical agent. Zinc rich unsatisfactory for acid or alkaline conditions unless top coated. Assure satisfactory adhesion top coats.
    Zone

    4

    		 Special conditions. For painting galvanized steel, mildew, temporary protection, rustproofing, painting welds, abrasion resistance, contact paint company coating specialist for specific conditions and advice.

    * These zones are intended as specific exposure zones of the structure under consideration, rather than geographic localities. Severity of exposure can change sharply over very short distances due to factors of wind, spray, condensation, fumes and deicing chemicals.