1、SSPC-PS Guide 8.00 April 1, 2002 Editorial Revisions November 1, 2004 SSPC: The Society for Protective Coatings PAINTING SYSTEM GUIDE NO. 8.00 Guide to Topcoating Zinc-Rich Primers 1. Scope 1.1 Thisguide covers the selection and application (includ- ing surface preparation) of topcoats to surfaces c
2、oated with a zinc-rich primer. Both organic and inorganic zinc-rich primers are included. 1.2The guide does not cover the selection and application of the zinc-rich primer. 2. Description 2.1 Thisguide representsgood practice fortopcoating zinc- rich primers as determined by industry consensus. It m
3、ay not be applicable to all materials and conditions encountered. 2.2 Zinc-rich primers are topcoated to provide extended exterior durability in severe exposures; to improve color, gloss, and appearance; and to provide resistance to specific condi- tions such as highly acidic or highly alkaline envi
4、ronments. 2.3 Information on the use of one coat zinc-rich painting systems is provided in SSPC-PS 12.01. 2.4 DEFINITIONS: Bubbling: Air bubbles or solvent vapor bubbles found temporar- ily in the wet film of a coating or permanently in the dry film. Burnish: To polish or rub to a smoother or glossi
5、er surface. Craters: Small, rounded depressions in a coating film that do not expose the previous coat or the substrate. Curing (cure): The process by which a coating changes from a liquid state into a dry stable solid protective film. Curing of a coating may involve chemical reaction with oxygen, m
6、oisture, or chemical additives, or the application of heat or radiation. Cure time and dry time are generally not identical, as curing may continue after the coating is dry to touch. Dry spray: A rough, powdery, noncoherent film produced when an atomized coating partially dries before reaching the i
7、ntended surface. Intermediate coat: One or more coats of paint between the primer and the finish coat, sometimes called a mid-coat. If a mist coat is necessary to reduce bubbling, it is considered pari of the intermediate coat in a 3-coat system. Microcracking: Cracksinthedriedfilmthatarevisibleonly
8、 under magnification and are not detrimental to zinc-rich coatings. Mist coat:Also called afog coat or tackcoat. Athin coat applied as a mist of spray and used to achieve maximum adhesion of a new coat to an existing partially or fully cured coat. Mudcracking: Acoating defect resembling the irregula
9、r crack- ing of drying mud that typically arises during the curing of a relatively inflexible coating applied too thickly. Pinhole: Aholiday or discontinuitythat extendsentirelythrough a coating film, approximately the size of a pin; caused by solvent bubbling, moisture, or foreign particles. Primer
10、: The first full coat of paint applied to a surface when a multicoat system is being used. Reduced coat: A thin coat containing extra solvent, used to reduce bubbling, cratering, and pinholing of the finish coat applied over it. Sealer: A coating that provides a seal against absorption or bleeding.
11、Tie coat: A paint formulated specifically to provide a transition from a primer or undercoat to a finish coat. Tie coats are used to seal the porous surface (e.g., zinc-rich primer) to bond ge- nerically different types of coatings or to improve the adhesion of a succeeding coating. Topcoat: The las
12、t coating material applied in a coating system, specifically formulated for aesthetics and/or environmental resistance. Also referred to as a finish coat. If a mist coat is necessary to reduce bubbling, it is considered pari of the topcoat in a two-coat system. Zinc-rich primer: Zinc-rich primers as
13、 referenced in this guide are broadly defined as primers containing high levels of zinc such as SSPC-Paint 20 or SSPC-Paint 29. Definitions taken from the SSPC Protective Coatings Glossary SSPC publication 00-071. 5-39 SSPC-PS Guide 8.00 April 1, 2002 Editorial Revisions November 1, 2004 Zinc salts:
14、 White oxidation products (oxide, hydroxide, carbonate) of zinc formed on galvanized and other zinc coated surfaces. (Defined as “white rust” in PCG.) 3. Referenced Standards 3.1 SSPC AND JOINT STANDARDS: PA 1 PA 2 Paint 20 Paint 27* Paint 29 PS 12.01 SP 1 SP 12NACE No. 5 Shop, Field and Maintenance
15、 Painting of Steel Measurement of Dry Coating Thickness With Magnetic Gages Zinc-Rich Primers (Type I- Inorganic however, an appearance as smooth as an automotive finish coat should not be expected. If the primer has not been identified, select a primer based on the information in Section 4.2 and Ta
16、ble 1. 5. Surface Preparation and Repair of Primer 5.1 IDENTIFY AND REPAIR DEFECTS: The zinc-rich primer to be topcoated must be inspected for defects, and the defects must be repaired before the application of a topcoat. These defects include rust, low dry film thickness (DFT), high DFT, dry spray,
17、 mudcracking, foreign matter, and loose zinc salts. The coating manufacturer should supply recommended procedures for the repair of defects. Some general guidance is provided below. 5.1.1 Rust: Rust resulting from the corrosion of the steel substrate and loose deposits of rust stain must be removed.
18、 However, light adherent stain, resulting from rust running from * This paint contains chromate pigments. Users are urged to follow all health, safety, and environmental requirements in applying, handling, or disposing of these materials. ASTM International, 100 Barr Harbor Drive, West Conshohocken,
19、 PA 19428-2959. 5-40 SSPC-PS Guide 8.00 April 1, 2002 Editorial Revisions November 1, 2004 Table 1 - Acceptability of PrimerITopcoat Combinations1 Inorganic Zinc-Rich Primer Organic Zinc-Rich Primer Thermoplastic (Solvent Thermoset Type II B2 Post Cure Self-cure Self-cure Type 1A2 Type 1 B2 Type 1C2
20、 Evaporation Cure) (Chemical Cure) Topcoat Water-Borne Water-Borne Solvent-Borne Type HA2 Vinyl Epoxy3 Water-Borne Acrylic Chlorinated Rubber Polyurethane4 Phenolic Silicone Alkyd Epoxy Ester Silicone Alkyd Oil Urethane Oil Phenolic Linseed Oil Coal Tar Epoxy Vinyl Alkyd B A B B B D A D D D D D D B
21、D B A B B B D B D D D D D D B D B A B A B D A D D D D D D B D Generally used directly on zinc with no difficulties. Some formulations can be used directly; others require a tie-coat. Lifting of zinc-rich primer is likely to occur unless well-aged or the primer is vinyl. Not recommended unless tie co
22、at is added. Not recommended. C E B A B D B D D D D D D E D B A A A B D B D D D D D D A D These are general guidelines, not hard-and-fast rules. This list is not exhaustive. Always consult the manufacturers recommendations. * See SSPC-Paint 20 (Section 2.3). Chemical cure such as epoxy amine, epoxy
23、polyamide, epoxy polyamine, or epoxy phenolic. Must be oil-free and alkali-resistant. 3 SSPC-PS Guide 8.00 April 1, 2002 Editorial Revisions November 1, 2004 adjacent rusted areas, need not be removed. As there are many different acceptable repair procedures, the manufacturers recommendations should
24、 be considered. 5.1.2 Low Dry Film Thickness: The dry film thickness should be measured in accordance with SSPC-PA 2. Low dry film thicknessshould be repaired before topcoating, based upon the recommendations of the zinc-rich primer manufacturer. 5.1.3 Excessive Dry Film Thickness: Excessive film th
25、ickness above maximum acceptable levels should be repaired according to the recommendations of the zinc-rich primer manufacturer. This may involve removal of excessive thickness only or complete removal and reapplication of the primer. The amount of zinc-rich primer that is excessive is highly depen
26、dent on the formulation, the environment, the require- ments for use, and the curing conditions. Based on a survey of suppliers, specifiers, and applicators, the most common value for maximum acceptable DFT is 125 to 150 micrometers (5 to 6 mils). For some zinc-rich primers, the maximum DFT may be a
27、s low as 90 micrometers (3.5 mils), while for certain organic zinc-rich primers a maximum DFT of 250 micrometers (10 mils) is acceptable. 5.1.4 Dry Spray: Paint that exhibits dry spray (defined in Section 2.4) has been improperly applied. The dry spray should be totally removed. Depending on the ext
28、ent of coating removal, the primer may have to be reapplied. It may be necessary to change types of thinners to retard solvent evaporation. A piece of wadded-up screen wire or screening attached to a wooden block may be used to remove dry spray. Awooden block may be shaped to fit the structural memb
29、ers involved. Tightly adhering zinc coats should not be removed. 5.1.5 Mudcracking:Thiscondition (defined in Section 2.4) usually resultsfrom excessive dry film thickness in a single coat. Mudcracking shall be corrected according to the manufacturers recommendations. This defect can occur anywhere,
30、but it is most likely to occur at inside corners and fillet welds. It is also possible that application at the correct thickness but under hot, dry, windy conditions can result in mudcracking. 5.1.6 Microcracking: Microcracks (defined in Section 2.4) that are visible only with magnification have no
31、effect on performance. 5.1.7 Foreign Materials: Dirt, mud, grease, oil, and other foreign materials must be removed prior to topcoating. Usually detergent and solvent cleaning, in accordance with SSPC- SP 1, are sufficient. However, some detergents may react with zinc to form a deleterious alkaline
32、byproduct. A final washing with clean (cleanser-free) water is always necessary.The manufacturer of the primer and the topcoat may be contacted for specific recommendations. and amounts of surface salts depending upon exposure. Loose zinc salts must be removed prior to topcoating, as they may have d
33、etrimental effects on the coating system. Such salts may be removed by fresh water washing and scrubbing. To achieve satisfactory results when topcoating post-cured inorganic zinc primers, it is important to clean the cure residue from the sur- face before applying topcoats. Normally, this is done b
34、y water washing. If pressure washing, perform a trial over a test area to ensure the primer is not degraded or reduced in thickness. Low pressure water cleaning (LP WC), defined in SSPC-SP 12 as cleaning performed at pressures less than 34 MPa (5000 psi), is usually sufficient. Ensure wash water is
35、clean and free of contaminants that could cause intercoat adhesion problems. Manual scrubbing with a stiff, non-metallic brush, while effec- tive, is labor intensive. Adherent zinc salts need not be removed for atmospheric exposure service. Under immersion service, however, water- soluble zinc salts
36、, even if adherent, may cause blistering or other problems when the zinc-rich primer is topcoated. Such water-soluble salts should be removed by thorough fresh water washing and scrubbing prior to the application of the topcoat. Atopcoated zinc-rich primer should not be used for immersion service un
37、less specifically recommended by the manufacturer. Wetted surfaces must be dry prior to the application of the topcoat. 5.2 ENSURE ADEQUACY OF CURE: The film formation of most zinc-rich primers includes solvent evaporation (drying) and chemical reaction (curing). To properly receive a topcoat, the p
38、rimer must be adequately dried and cured. Typically, 90% or more of the solvent will evaporate within the first 15 minutes. Completion of the chemical reaction of curing may require weeks or months; however, the primer may be able to receive a topcoat in a shorter period than required for full curin
39、g. Because of possible delamination and zinc splitting, topcoating undercured inorganiczincshould only be performed in accordance with the manufacturers recommendations. 5.2.1 Factors That Affect Cure: The following factors should be carefully considered prior to topcoating. 5.2.1.1 Humidity: Depend
40、ing on the type of primer, the time to cure can be significantly altered by humidity conditions. Many primers (especially solvent-borne inorganic) do not cure below 40% relative humidity. For some primers in low humidity environments, the application of a thin film of water after the initial drying
41、can be used to promote curing mechanisms. In humidity over 90%, some water-borne inorganic zinc primers do not properly cure. The manufacturer should be contacted regarding specific humidity requirements. 5.2.1.2 Temperature: The time to cure and the extent of cure for many zinc-rich primers is also
42、 affected by temperature. The manufacturer should be contacted for specific recommen- dations regarding temperature for curing prior to topcoating. 5.1.8 Zinc Salts: Zinc-rich primers may form varying types 5-42 SSPC-PS Guide 8.00 April 1, 2002 Editorial Revisions November 1, 2004 5.2.1.3 Dry Film T
43、hickness: For almost all zinc-rich primers, thicker films require longer cure times. Solvent-borne inorganic zinc-rich primers, especially the single- component types, are particularly sensitive to this effect. 5.2.2 Methods to Assess Adequacy of Cure: The recommendations of the zinc-rich primer man
44、ufacturers are the most important factors in determining the adequacy of cure of a zinc-rich primer. Some general methods for determining zinc-rich primer cure are listed below: 5.2.2.1 Solvent Rub: This method is only applicable to ethyl silicate zinc-rich primers that do not have butyl or other or
45、ganic modifiers. As described in ASTM D 4752, a clean rag soaked in methyl ethyl ketone (or other suitable solvent) is rubbed over the primer 50 times. No residue, or only traces of residue on the rag, indicates adequate cure. 5.2.2.2 Coin Rub: Rub the primer with the edge of a coin; if the film bur
46、nishes, it is acceptable. Some zinc-rich primers that do not burnish may still be acceptable for topcoating. When in doubt, consult the coating manufacturer. 5.2.3 Correcting Undercured Inorganic Zinc Primer 5.2.3.1 If a solvent-borne inorganic zinc-rich primer is undercured due to insufficient humi
47、dity, proper curing may be achieved by a longer curing time or by curing at a high humidity level achieved through natural weather variations or artificial means, such as spraying the primed surface with water. 5.2.3.2 Undercured water-borne inorganic zinc-rich primers may be corrected by furnishing
48、 clean, dry air to aid in water evaporation. In some cases, an acidic post-curing solution may be applied, followed by thorough rinsing. 5.2.3.3 If it is determined that the inorganic zinc-rich primer is not cured and cannot attain a satisfactory condition for topcoating, the zinc-rich primer must b
49、e removed, and a new primer must be applied. This condition is generally due to excessive solvent retention or insuff icient reaction of the vehicle with oxygen, moisture, or catalyst. The manufacturer should be contacted regarding specific humidity requirements. 5.2.4 Correcting Undercured Organic Zinc Primer 5.2.4.1 If it is determined that an organic thermoplastic zinc primer is undercured due to insufficient solvent evaporation, proper curing may be achieved by a longer curing time. The coating manufacturer should be contacted for recommenda- tions. 5.2.4.2 If it is determined that
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