1、SSPC-PA Guide 11August 1, 20081SSPC: The Society for Protective CoatingsPainT aPPliCaTion Guide no. 11Protecting edges, Crevices, and irregular Steel Surfaces by Stripe Coating1. ScopeThis guide discusses the technique called “stripe coating” or “striping” as a way of providing extra corrosion prote
2、ction measures on edges, outside corners, crevices, bolt heads, welds, and other irregular steel surfaces, including optional surface preparation techniques for sharp edges to improve coating performance. Some details, including the advantages and limitations of specific methods of obtaining additio
3、nal coating thickness, are described to assist the specification writer in assuring that the project specification will address adequate corrosion protection. 2. Background2.1 Sharp edges, outside corners, crevices, and welds are often sites where coating failures and corrosion begin due to reduced
4、coating barrier protection (see Figure 1). Reduced coating thickness on these surfaces can be a result of applica-tion of inadequate amounts of coating, but the surface tension of the wet coating or coating shrinkage during the drying/curing process can also pull the film away from edges. Sharp edge
5、s resulting from cutting, burning, and shearing are especially difficult to coat adequately. Bolt threads combine the adverse effects of sharp edges and crevices. 2.2 The solutions to this problem include (1) eliminating the irregularities by grinding, mechanical sanding, or filing; (2) applying a t
6、hicker protective coating film to problem areas such as outside edges, pitted areas, and bolts; (3) applying a penetrating and/or good wetting coating to crevice areas and/or inside corners; or a combination of (1) and (2). In some cases, both grinding and stripe coating may be specified for a given
7、 area. The option of choice is usually determined by the best cost/benefit ratio. Factors that enter into the cost/benefit analysis of edge protection include the severity of the exposure environment, interference with production during maintenance, and difficulty in accessing the structure to perfo
8、rm maintenance painting. These factors are unique to each project.2.4 Sharp edges may be avoided at the prefabrication stage or remedied by grinding after construction, but at a much higher cost. When filing edges, care should be taken to assure that additional sharp edges are not created while modi
9、fying the original edge. It is generally much less expensive and easier to grind or chamfer edges in the shop than in the field.2.5 Additional coating thickness can be obtained by applying an extra layer of coating to the areas requiring addi-tional protection (the “stripe coat”). In some cases, the
10、 “stripe coat” may be followed by the application of a full coat of a coating with edge retention properties.3. Preparing outside Corners and Sharp edges3.1 Outside corners and sharp edges can be ground such that the edge is rounded to a 1.6 to 3.2 mm (1/16 to 1/8 inch) radius. An alternative to rou
11、nding is to chamfer the edge to a specified measurement, so instead of one 90-degree corner, there are two 135-degree corners (see Figure 2). The width of the flat ground area, i.e. the distance between the two 135-degree corners, is usually 1.6 to 3.2 mm (1/16 to 1/8 inch). Paint will not pull away
12、 from a 135-degree corner as much as from a 90-degree corner, resulting in more overall coating thickness. Further, subsequent abrasive blast cleaning provides some radius to the 135-degree corners.Figure 1. Schematic showing how coating thick-ness is reduced at a sharp edge.90 135 Chamfer Figure 2.
13、 Chamfering reduces a sharp 90 corner to two 135 corners.Poor Coating Ideal SSPC- PA Guide 11August 1, 200823.2 The increased performance resulting from rounding or chamfering edges depends on the service environment, the generic type of coating and on the radius or size of the chamfer. One research
14、 study1showed that rounding or chamfering had no measurable effect if an ethyl silicate inorganic zinc-rich (IOZ) coating was applied over blast cleaned steel. Therefore, there is no need to go to the added expense of grinding edges if ethyl silicate IOZ coating is specified. However, the same study
15、 indicated that rounding or chamfering edges before application of an alkyd or organic zinc-rich primer improved the coatings performance. Even with edge rounding or cham-fering, it is necessary that proper spray application techniques be used.2,3The use of coatings with edge-retention properties (s
16、ee Section 4.7.3) may also minimize or eliminate the need for edge grinding.3.3 Edge grinding is best done in the shop before blast cleaning where the work is accessible. A sample statement in a specification may read, “All corners and edges shall be rounded to a 2 mm radius (1/16 inch) or ground to
17、 a 2 mm (1/16 inch) chamfer.” In addition to edges, rough surfaces are ground smooth. Flame cut edges are normally ground over their entire surface to remove the hardened surface layer resulting from flame cutting and permit creation of a profile. For immersion service and other services where incre
18、ased corrosion can be expected, very rough welds should always be ground because many coatings cannot adequately protect them. NACE Standard RP0178 “Standard Recommended Practice: Fabrication Details, Surface Finish Requirements, and Proper Design Considerations for Tanks and Vessels to Be Lined for
19、 Immersion Service“4provides guidance on finish requirements for welds.4. Stripe Coat application A “stripe coat” is a coat of paint applied only to edges, welds, outside corners, bolt heads and threads, and crevices, either before or after a full coat is applied to the entire surface. These areas a
20、re difficult to coat due to coating shrinkage during cure. Shrinkage pulls coating away from sharp edges, resulting in low dry film thickness. Wet coating may flow over rather than into crevices. This results in a temporary creating bridge over the crevice that disappears as the coating cures and sh
21、rinks, leaving the crevice interior unprotected. Both areas are susceptible to premature corrosion. The practice of applying a “stripe coat” to such areas is frequently referred to as “striping.”The project specification normally specifies the areas are to be striped, how to apply the stripe coat (b
22、rush or spray), in what order to apply the stripe coat (before or after the full coat), which coats are to be striped, whether tinting of the stripe coat or use of another color of coating is required, and information on the drying/curing requirements between application of the stripe and full coat
23、(may defer to the coating manufacturer). 4.1 areas to Stripe: The facility owner normally determines whether stripe coating is warranted for some or all areas of the particular project. Sometimes only specified areas of outside corners, edges, welds, and crevices are striped. Situations where stripe
24、 coating is usually warranted are:Immersion service (e.g., interiors of fuel or water storage tanks; ship hulls)Areas with high corrosion rates (e.g. cooling towers and splash zones)Areas where access is difficult (e.g., towers, some bridges).Built up members (e.g., lattice bars and boxes) Situation
25、s where stripe coating may not be cost effective are:Non-critical components (e.g., interior wall in living space on a ship)Mild exposure (e.g., SSPC environmental exposure zones 1A and 1B(1),5Inside corners (can be adequately covered using proper spray technique)Areas that are usually stripe coated
26、 include:Crevices Plate and sharp edges Plate seams Back-to-back angle seams (built up members) Pitted steel Bolt heads and nuts Rivet heads Welds Other sharp discontinuities Outside corners Stripe coating is most effective on edges that are either rounded or chamfered by grinding (see Section 3). 4
27、.2 How to Stripe4.2.1 Brushing is the preferred method of applying stripe coats. Unless otherwise specified, SSPC-PA 1 requires brush application of coatings to cracks, crevices, blind areas of all rivets and bolts, and all areas of limited access.(2),6Brush application provides the highest quality
28、stripe coat on most surfaces because it works the paint into the pores and crevices and allows the greatest control over the boundaries of the painted area. The coating should be allowed to dry to recoat according to the product data sheet before the full prime coat is applied. Coating manufacturers
29、 will usually provide guid-ance on the amount of dry time required before application of (1)SSPC Environmental Zone 1AInterior, normally dry (or tem-porary protection). Very mild (oil base paints now last six years or more). SSPC Environmental Zone 1BExterior, normally dry (includes most areas where
30、 oil base paints now last six years or more)4(2)SSPC-PA 1, Section 7.4.6.SSPC-PA Guide 11August 1, 20083subsequent coatings. Table 1 compares application of stripe coats by brush or spray.4.2.2 When applying a stripe coat, the brush strokes should run parallel to, not across, edges and crevices. Var
31、iations in pressure of the brush during application can result in thin film on sharp edges and accumulation of coating on adjoining perpendicular surfaces. To assure adequate film build on sharp edges, multiple brush applications may be necessary. Bolt heads and nuts should be striped in a circular
32、brush motion. Striping should extend at least 3 centimeters (cm) (1 inch) from edges and other irregular surfaces. Round or oval brushes are generally considered most suitable for rivets, bolts, irregular surfaces, and rough or pitted steel.4.2.3 SSPC-PA 1 does not permit rollers to be used to apply
33、 coatings on irregular surfaces, such as rivets, bolt heads, crevices, welds, corners or edges unless otherwise specified.(3) If the project specification permits the use of rollers for stripe coat application in these areas, PA 1 requires that the coating be brushed out to form a continuous and unb
34、roken film over these surfaces.4.2.4 Spraying the stripe coat can reduce the amount of time spent in brush application, but the painter has less control over the precise boundaries of the area being coated. Spray may be the best method for applying stripe coats using coatings with edge retention pro
35、perties or zinc-rich coatings, or for applying a stripe coat to edges that have already been painted. The applicator should spray the coating along the flat surface, not directly at the edge itself, to prevent the atomiza-tion pressure from pushing the coating away from the edge that is to be protec
36、ted.3 SSPC-PA 1, Section 7.3.4. 4.2.5 When striping with a coating that is prone to pigment settling, such as zinc-rich primers, the coating in the pot should be constantly agitated, unless otherwise recommended by the coating manufacturer. 4.2.6 When striping with a multi-component thermosetting co
37、ating, the curing reaction will eventually progress to the point where the coating fails to wet the surface and level adequately. Since high temperatures often accelerate curing rates, this effect is most pronounced in hot weather. Heat of reaction (exotherm) may also be generated when multiple comp
38、onents of thermo-setting coatings are mixed, so it may be more cost effective to use smaller kits (e.g., 1-liter or 1-gallon units) rather than 5-liter or 5-gallon units. Note that most coating manufacturers prohibit mixing of partial kits. Thinning should not be used to restore the flow properties
39、of a multi-component coating that has increased viscosity from partial curing.4.2.7 It is good practice for the stripe coat to be a different color from the adjoining full coats or the bare steel surface. The color difference enables painters and inspectors to visually verify there are no misses, sk
40、ips, or thin spots in the stripe coat. In addition, the application can be verified after the fact (as necessary) for thickness and proper coverage using a destruc-tive thickness gage (i.e., Tooke Gage), a holiday detector, or (on some specialty coatings) ultraviolet light. 4.3 What Coats to Stripe:
41、 Usually, a stripe application is specified for the primer coating because of the importance of complete coverage. After a surface has received its first coat of paint, there is less probability of incomplete coverage of subsequent coats. Occasionally, multiple stripe coats may be required over roug
42、h hand welds. In critical areas, the specifier TaBle 1 CoMPaRiSon oF aPPliCaTion MeTHodSaPPliCaTion MeTHodadVanTaGeS diSadVanTaGeSBrush Best method to work paint into pores and crevicesMost complete coverage of rivet heads, bolt heads, nuts, and threadsBest control of extent of area coated Most labo
43、r-intensive method May pull incompletely cured paint from edge Spray Fastest method of application Least labor-intensive method May be best method of applying coatings with edge retention properties, zinc-rich coatings Quick method for applying a stripe coat to edges al-ready paintedMay bridge (rath
44、er than penetrate into) pores and crev- ices. Back-brushing needed to work wet coating into pits, crevices, and angles.More likely to miss areas in tight spaces, resulting in holidays.Builds coating thickness on larger area (control of area coated is difficult)More risk of overspray and dry spray SS
45、PC- PA Guide 11August 1, 20084may require that striping be done for two or more coats in a multi-coat system. Nevertheless, multiple coat striping is not commonly specified.4.4 When to Stripe - Before or after Full Coat4.4.1 Many coatings professionals feel that the stripe coat of the primer should
46、be applied first to maximize corro-sion protection. For example, it is easier to work stripe coats into crevices that have not been bridged by a spray-applied full coat. However, there are several reasons why the stripe coat could be applied after the first full coat. If the stripe coat is applied p
47、rior to primer application, there is opportunity for the surrounding clean, unprotected steel to deteriorate while the stripe coat is drying/curing. Another reason is that stripe coating after application of the full coat (particularly an inter-mediate coat) will fill in pinholes and other holidays
48、that have formed in the spray-applied coat. 4.4.2 If striping is specified for each coat in a multi-coat system, application of the stripe coat before the final finish coat will result in a more uniform finish.4.4.3 When painting a tank floor or other surface subject to foot or vehicle traffic, it m
49、ay be wise to delay application of the final stripe coat over the welds to minimize erosion of the coating by foot traffic or air hoses being dragged across the raised welds prior to the coating being put into service.4.4.4 The advantages and limitations of striping before and after the full coat are summarized in Table 2. Final deter-mination of the best sequence of stripe coats and full coats will depend on the coating systems being used and the specific requirements of each project.4.5 If the stripe coat is applied first, it is usually allo
