Surface Preparation

After the selection of a coating system, the most important factor to be considered is surface preparation. Surface preparation must be compatible with the primer and topcoating. Surface preparation is often the most costly phase of the corrosion prevention process for steel, averaging over half the cost of paint application exclusive of scaffolding.

Surface preparation should remove mill scale, rust, oil, grease, atmospheric materials, weld spatter, and old coatings. Surface preparation also provides an anchor pattern to allow the primer and following coats of paint to key into the surface for a good bond. Table 17 shows minimum surface preparation requirements for steel with commonly used coatings. Table 13 summarizes various methods of surface preparation used before painting and provides the Steel Structures Painting Council (SSPC) designation for each method.

Inspection of Surface. Before proper surface preparation and painting method can be applied, the condition of the surface must be determined. For new or previously uncoated steel surfaces, the Steel Structures Painting Council visual standard (SSPC-VIS1) defines the four rust grades of structural steel and contains colored photographs presenting surface preparation standards. Shop primed coated steel would be expected to have no rusting except for small areas abraded during handling.

However, most of the steel structures encountered have been coated previously. The coating layers may be concealing as much rust as is showing on the surface. The surface condition of a previously coated steel substrate may be classified in accordance with ASTM 610/SSPC-VIS2 for the degree of rusting, as shown in Table 19 (SSPC-VIS2 for Pictorial Representation of Rust Classification). The chalking, blistering, flaking, erosion, checking, and cracking of the coating may be classified by the corresponding ASTM visual standards. Instrumentation is available for determining the presence of pinholes, the adhesion of the coating film to the substrate, and the film thickness.

Table 19 Steel Structures Painting Council standard SSPC-VIS2 for pictorial representation of rust classification for surfaces to be painted

Paint system condition

Cleaning and painting recommended

Rust grades

Area of example, %

Nondeteriorated, 0 to 0.1% rust

Paint almost intact; some primer may show; rust covers less than 0.1% of the surface

Solvent clean (SSPC-SP1) entire repaint area, and spot prime, if necessary. If required to maintain film thickness or continuity, spot apply finish coat, then apply 38-51 .|,;m (1.5-2.0 mils) of finish coat over entire repaint area

10-8

*

Slightly to moderately deteriorated, 0.1 to 1% rust

Finish coat somewhat weathered; primer may show slight staining or blistering; after stains are wiped off, less than 1% of area shows rust; blistering, loose mill scale, or loose paint film

Spot clean (minimum SSPC-SP2) entire repaint area, and spot prime. If required to maintain minimum film thickness or continuity, spot apply finish coat, then apply 38-51 .'-'m (1.5-2.0 mils) of finish coat over entire repaint area

8-6

#

Deteriorated, 1 to 10% rust

Paint thoroughly weathered, blistered, or stained; up to 10% of surface is covered with rust, rust blisters, hard scale or loose paint film, very little pitting visible

Spot clean (minimum SSPC-SP2) entire repaint area, feather edges, and spot prime. If required to maintain film thickness or continuity, spot apply finish coat, then apply 38-51 ^m (1.5-2.0 mils) of finish coat over entire repaint area

6-4

' " ■ . *

Severely deteriorated, 10 to 50% rust

Large portion of surface is covered with rust, pits, rust nodules, and non-adherent paint. Pitting is visible

Clean (minimum SSPC-SP6) entire repaint area, feather edges, and spot or full prime. If required to maintain film thickness or continuity, spot apply finish coat, then apply 38-51 ^m (1.5-2.0 mils) of finish coat over entire repaint area

4-1

►•« .r rt' J

«

Totally deteriorated, 50 to 100% rust

Clean (minimum SSPC-SP6) entire repaint area and apply primer, intermediate, and finish coats over entire repaint area

1-0

Surface profile allows the coating to key into the metal substrate. Surface profile is determined by the abrasive material, including hardness, mass, and firmability, and the force with which the abrasive material impinges upon the surface. The selection of appropriate surface profile provides the bases for good primer adhesion. The coating formulator takes into account the viscosity of the primer, which allows the coating to fill the contact surface, the number of polar groups to come in contact with this surface, and the mechanical anchor or tooth that facilitates adhesion of the primer to the contact surface. Surface profile cannot expose the peaks of the metal once the primer is applied. Therefore, the abrasive blaster must use the correct angle of attack and distance from the work during the blasting operation. An abrasive-blasted surface profile of from 0.038 to 0.089 mm (1.5 to 3.5 mils), measured from the top of the highest peak to the bottom of the lowest valley, is used for most coating systems.

Surface inspection includes the determination of the condition of the surface before surface preparation and the results of the surface preparation. SSPC VIS1 provides a standard for surfaces before preparation and prepared surfaces. Surface comparison may also be made by using a surface-profile comparator and the NACE Standard TM-01-70, prepared by the National Association of Corrosion Engineers.

Structural design may limit access to the sections being prepared for painting. Access for abrasive blasting requires approximately 455 mm (18 in.) of clearance of the blast nozzle, although 150 mm (6 in.) can be sufficient if superficial cleaning can be tolerated. Sharp edges resulting from corrosion pits, deep gouges, or cut edges should be properly prepared. Pits and gouges over 3.18 mm (^ in.) deep should be filled with weld metal and ground flush to the surface or

prepared for painting with an epoxy grout. Cut edges should be chamfered to a 3.18 mm (^ in.) radius by grinding. Bolts

and rivets should be tight against the steel plates, not allowing crevices; areas surrounding bolts and rivets should be hand brushed with primer after the areas have been abrasive blasted. Discontinuous weldments or tack welds should be properly prepared. Continuous welds should be required, or an epoxy grout or caulk should be used to eliminate water accumulation. Where water may accumulate, in areas such as cross members joining channels and L sections that are directly exposed to the sky, weep holes are needed that are sufficiently large to allow water to drain and debris to be flushed through.

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