How To Topcoat Zinc-Rich Primers
Painting System Guide No. 8.00 – Guide to Topcoating Zinc-Rich Primers
1. Scope
1.1 This guide covers the selection and application (including surface preparation) of topcoats to surfaces coated with a zinc-rich primer. Both organic and inorganic zinc-rich primers are included.
1.2 The guide does not cover the selection and application of zinc-rich primers.
2. Description
2.1This guide represents good practice for topcoating zinc-rich primers as determined by industry consensus. It may 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 conditions such as highly acidic or highly alkaline environments.
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 temporarily in the wet film of a coating or permanently in the dry film.
Burnish: To polish or rub to a smoother or glossier 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, moisture, 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, non-coherent film produced when an atomized coating partially dries before reaching the intended 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 part of the intermediate coat in a 3-coat system.
Microcracking: Cracks in the dried film that are visible only under magnification and are not detrimental to zinc-rich coatings.
Mist coat: Also called a fog coat or tack coat. A thin 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: A coating defect resembling the irregular cracking of drying mud that typically arises during the curing of a relatively inflexible coating applied too thickly.
Pinhole: A holiday or discontinuity that extends entirely through a coating film, approximately the size of a pin; caused by solvent bubbling, moisture, or foreign particles.
Primer: 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.
Tie coat: A paint formulated specifically to provide a transition from a primer or undercoat to a fi nish coat. Tie coats are used to seal the porous surface (e.g., zinc-rich primer) to bond generically different types of coatings or to improve the adhesion of a succeeding coating.
Topcoat: The last 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 part of the topcoat in a two-coat system.
Zinc-rich primer: Zinc-rich primers as referenced in this guide are broadly defined as primers containing high levels of zinc such as SSPC-Paint 20 or SSPC-Paint 29.
Zinc salts: White oxidation products (oxide, hydroxide, carbonate) of zinc formed on galvanized and other zinc coated surfaces. (Defined as “white rust” in PCG.)
Definitions taken from the SSPC Protective Coatings Glossary [SSPC publication 00-07].
3. Referenced Standards
3.1 SSPC AND JOINT STANDARDS:
PA 1 Shop, Field and Maintenance Painting of Steel
PA 2 Measurement of Dry Coating Thickness With Magnetic Gages
Paint 20 Zinc-Rich Primers (Type I–Inorganic & Type II–Organic)
Paint 272 Basic Zinc Chromate-Vinyl Butyral Wash Primer
Paint 29 Zinc Dust Sacrificial Primer, Performance-Based
PS 12.01 One Coat Zinc-Rich Painting System
SP 1 Solvent Cleaning
SP 12/NACE No. 5 Surface Preparation and Cleaning of Steel and Other Hard Materials by High-and Ultra-High Pressure Waterjetting Prior to Recoating
3.3 AMERICAN SOCIETY FOR TESTING AND MATERIALS (ASTM) STANDARD:
D 3359 Standard Test Methods for Measuring Adhesion by Tape Test
D 4541 Standard Test Method for Pull-Off Strength of Coatings Using Portable Adhesion Testers
D 4752 Standard Test Method for Measuring MEK Resistance of Ethyl Silicate (Inorganic) Zinc-Rich Primers by Solvent Rub
4. Selection of Topcoat
4.1 IDENTIFY PRIMER: Proper selection of topcoats requires identification of the generic (chemical) type of primer. Often the specific product name is needed as well. First, determine the class of primer in accordance with SSPC-Paint 20, which categorizes zinc-rich primers by the type of vehicle and curing mechanism. If the primer has already been applied, record the date of application, manufacturer’s name, product identification, batch number, and manufacturing date. If the primer has been selected but not applied, record the name of the manufacturer and the product designation. It is good painting practice to obtain the primer, intermediate coat (if any), and topcoat from the same manufacturer.
4.2 COMPATIBILITY CONSIDERATIONS: When topcoating zinc-rich primers, consideration must be given to the fact that zinc is being coated, not steel. Coatings based on drying oils, such as most oil and alkyd types, tend to react with the zinc metal to form zinc soaps, resulting in loss of adhesion and peeling of the topcoat. In general, coatings that have poor alkali resistance should not be applied directly to zinc-rich primers. Coatings with oil-containing vehicles such as oil paints, alkyd enamels, vinyl alkyds of the oxidizing type, long oil silicone alkyds, and epoxy esters should not be applied directly to zinc-rich primers.
4.3 SELECT COMPATIBLE TOPCOAT: Table 1 lists combinations of commonly used zinc-rich primers and topcoats. For each combination, this table indicates the degree of acceptability, ranging from A (used directly) to E (not recommended). Although all of the generic coating types listed in Table 1, except alkyds, have been successfully applied as topcoats over zinc-rich primers, combining different manufacturers’ products may adversely affect coating performance. The table reflects the application of the topcoat directly to zinc-rich primers, not its use as a final coat in a multi-layered coating system. No coating should be applied to a zinc-rich primer unless recommended by the manufacturer for that purpose. If the primer type has previously been identified, select a topcoat based on the information in Sections 4.1, 4.2, and Table 1.
Other factors that may influence selection of topcoat and primer/topcoat combinations are: specific durability of topcoats in intended exposure environments, appearance of the finished coat, recoatability, heat resistance, and cost. Information on these factors may be found in the SSPC Painting Manual, Volume 1, Good Painting Practice, as well as in other technical publications, and from coating manufacturers. The general appearance of a properly applied zinc-rich topcoated system is acceptable for typical industrial applications; however, an appearance as smooth as an automotive finish coat should not be expected. If the primer has not been identifi ed, select a primer based on the information in Section 4.2 and Table 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, 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. However, light adherent stain, resulting from rust running from
2 This paint contains chromate pigments. Users are urged to follow all health, safety, and environmental requirements in applying, handling, or disposing of these materials.
5.1.2 Low Dry Film Thickness: The dry film thickness should be measured in accordance with SSPC-PA 2. Low dry film thickness should be repaired before topcoating, based upon the recommendations of the zinc-rich primer manufacturer.
5.1.3 Excessive Dry Film Thickness: Excessive film thickness 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 dependent on the formulation, the environment, the requirements 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 as 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 extent 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.A wooden block may be shaped to fit the structural members involved. Tightly adhering zinc coats should not be removed.
5.1.5 Mudcracking: This condition (defined in Section 2.4) usually results from excessive dry film thickness in a single coat. Mudcracking shall be corrected according to the manufacturerʼs recommendations. This defect can occur anywhere, 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 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 SSPCSP 1, are sufficient. However, some detergents may react with zinc to form a deleterious alkaline 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.
5.1.8 Zinc Salts: Zinc-rich primers may form varying types and amounts of surface salts depending upon exposure. Loose zinc salts must be removed prior to topcoating, as they may have detrimental 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 surface before applying topcoats. Normally, this is done by 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 clean and free of contaminants that could cause intercoat adhesion problems. Manual scrubbing with a stiff, non-metallic brush, while effective, is labor intensive.
Adherent zinc salts need not be removed for atmospheric exposure service. Under immersion service, however, water-soluble zinc salts, 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. A topcoated zinc-rich primer should not be used for immersion service unless 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 primer 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 curing. Because of possible delamination and zinc splitting, topcoating undercured inorganic zinc should only be performed in accordance with the manufacturerʼs recommendations.
5.2.1 Factors That Affect Cure: The following factors should be carefully considered prior to topcoating.
5.2.1.1 Humidity: Depending 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 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 affected by temperature. The manufacturer should be contacted for specific recommendations regarding temperature for curing prior to topcoating.
5.2.1.3 Dry Film Thickness: 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 manufacturers 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 organic 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 burnishes, 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 humidity, 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 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 be removed, and a new primer must be applied. This condition is generally due to excessive solvent retention or insufficient 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 recommendations.
5.2.4.2 If it is determined that an organic thermoset zinc primer is not cured properly due to either too much or too little catalyst or hardener, the coating must be removed and more zinc-rich primers must be applied.
6. Application of Topcoats
6.1 GENERAL
6.1.1 The surface of the zinc-rich primer to be topcoated should be free of loose zinc salts, dry spray, foreign materials, and other defects described in Section 5.1. Paints meeting the requirements of SSPC-Paint 29 have topcoating properties comparable to paints meeting the requirements of SSPC-Paint 20.
6.1.2 The primer should be free of moisture. Certain topcoats have a high sensitivity to moisture. For example, amine cured epoxy coatings may show large blisters if any moisture remains in the pores of the zinc primer at the time of topcoat application.
6.1.3 The primer should be adequately cured as described in Section 5.2, based on the requirements of the primer manufacturer.
6.1.4 In general, the topcoat application should be in accordance with SSPC-PA 1. The manufacturer should be consulted to see if a mist coat or a reduced coat is required.
6.1.5 Factors that affect the acceptability and performance of a topcoated zinc-rich primer include porosity of primer, extent of topcoat bubbling, and adhesion of topcoat. General guidelines are given below.
6.2 POROSITY OF PRIMERS: A zinc-rich primer consists of particles of zinc metal bound to each other by a suitable inorganic or organic binder. The zinc dust content of the dry film may vary from 65 to 95 percent of the total weight. The small amount of binder results in a very high pigment volume concentration for both organic and inorganic zinc-rich primers, resulting in a porous coating. Porosity becomes more pronounced if the zinc-rich primer is dry sprayed, the weather is hot, the equipment is improperly adjusted, or the applicator failed to apply the zinc-rich primers in a full wet coat. These conditions contribute to pinholing, bubbling, and cratering of the topcoat, the severity of which may vary with the type of binder used.
6.3 TOPCOAT BUBBLING AND PINHOLING
6.3.1 Origin of Bubbles and Pinholes: If the topcoat is capable of penetrating the pores and forming a film, this penetration may provide a base for initial adhesion. However, when organic topcoats and their solvents penetrate into the pores of the primer, they may also force air up through the wet film of paint. A bubble or crater results if the surface has “skinned over” (i.e., a semi-solid membrane has formed on the surface while the coating below is still liquid) or has lost its ability to flow. Pinholes may also occur under these circumstances. (See Section 2.4 for definitions of bubbling and pinholing.)
6.3.2 Occurrence of Bubbles and Pinholes: Generally, the self-curing inorganic zinc primers have a greater tendency to cause bubbling or pinholing in the topcoat than post-curing inorganic or organic zinc-rich primers. High-build coatings specially formulated for application over zinc-rich coatings can reduce bubbling and other application problems. Low-build coatings, especially gloss or semigloss coatings, have a greater tendency for bubbling or pinholing. Generally, the greater the thickness of the zinc-rich primer, the greater the tendency for bubbles and pinholes to form in the topcoat.
6.3.3 Repair of Bubbles and Pinholes: Bubbles, pinholes, or craters are unsightly and should be avoided using the methods in Section 6.4. They can be repaired by sanding the affected areas, followed by reapplication of the topcoat. Sweep blasting can be effective. However, use of a relatively friable or less hard abrasive and/or reduced nozzle pressure will help avoid excessive ablation of the coating. If methods other than sanding are to be allowed, the contracting parties should agree in advance on the limits of the process and the type of abrasive. When properly repaired, bubbles and pinholes do not affect the performance of the system. If left unrepaired, the performance may be affected if the environment is characterized by aggressive chemical fumes or chemical contaminants, combined with high humidity and condensed moisture.
6.4 MINIMIZING BUBBLING AND PINHOLING
6.4.1 Some manufacturers recommend the application of a mist coat (see Section 2) of the topcoat to reduce bubbles, pinholes, and craters, while others recommend a reduced coat (see Section 2) of the topcoat. Both are followed by a full wet coat of the topcoat. The application of the topcoat in two coats (a mid coat/full coat or a thinned coat/full coat) rather than a single heavy coat will reduce the chance of surface defects, such as pinholes and bubbles.
6.4.2 Other manufacturers recommend using a tie coat, such as vinyl wash primer (SSPC-Paint 27), to condition the substrate. The acid component of the wash primer reacts with the zinc and allows the wash primer to bond to the zinc-rich primer. This may be followed by a topcoat of any compatible type. Other specially formulated tie coats may also be used to promote adhesion for topcoats.
6.5ADHESION OF TOPCOATED SYSTEMS: Three common methods of measuring adhesion of topcoated zinc-rich primer systems have been identifi ed. However, no consensus for minimally acceptable adhesion ratings could be established for any of these methods. The user should consult the coatings manufacturer regarding appropriateness and interpretation of adhesion test results. The three methods for measuring adhesion are:
6.5.1 X-Cut Tape Test (ASTM D 3359 – Method A): An X-cut is made in the film to the substrate, pressure sensitive tape is then applied over the cut and removed. Adhesion is assessed quantitatively on a scale of 0 to 5 by comparison with descriptions.
6.5.2 Cross Cut Tape Test (ASTM D 3359 – Method B): A lattice pattern with either 6 or 11 cuts in each direction is made in the film to the substrate, pressure sensitive tape is applied over this area, and then removed. Adhesion is assessed by comparison with descriptions and illustrations on a scale of 0 to 5. This method is not recommended for film thicknesses greater than 125 micrometers (5 mils).
6.5.3 Portable Adhesion Testers (ASTM D 4541): A dolly is glued to the coating surface, and then pulled off. The force required to remove the dolly, as well as the nature of the disbonding, are evaluated. This test provides a quantitative measure of the pull-off strength, but the significance of the adhesion values has yet to be established.
6.6 TOUCH-UP AND REPAIR OF TOPCOATED ZINCRICH PRIMER: Touch-up and repair of a topcoated zinc-rich primer may be required when the topcoat is damaged, when areas intended to be topcoated are missed, or when the total dry fi lm thickness is insuffi cient. The methods of touch-up and repair depend upon the specific generic type of the topcoat (e.g., epoxy, vinyl, or urethane) and the time interval between the original topcoating and the touch-up and repair operation. The new coating should be applied to provide uniform appearance with the existing coating, including feathering of edges. The surface may require cleaning by solvent, detergent, or pressurized water to remove dirt, grease, and other surface contaminants. Two-component, chemically curing topcoats may require mechanical abrading to provide adequate surface roughness, even when reapplied over the same generic type of coating. The topcoat manufacturer should be consulted for the procedures for specific coatings.
7. Inspection
7.1 Unless otherwise specified in the procurement documents, the contractor and the material supplier are responsible for quality control to assure that the requirements of this document are met. All work and materials supplied under this guide are also subject to timely inspection by the purchaser or his authorized representative. The contractor or the coating manufacturer should correct such work or replace such material as is found defective under this guide. In case of a dispute, an arbitration or settlement procedure established in the procurement documents (project specification) shall be followed. If no arbitration or settlement procedure is established, then a procedure mutually agreeable to purchaser, material supplier, and contractor shall be used.
7.2 Samples of paints under this painting system may be requested by the purchaser, and should be supplied upon request along with the manufacturer’s name and identification for the materials. Samples may be requested at the time the purchase order is placed. Samples taken at the jobsite should be from previously unopened containers.
7.3 Unless otherwise specified, the methods of sampling and testing should be in accordance with ASTM D 3925.
8. Disclaimer
8.1 This Guide is designed to describe, review, or analyze new or improved technology and does not meet the definition of a standard as defined by SSPC. A guide differs from a standard in that it is not suitable for referencing in a specification or procurement document.
8.2 While every precaution is taken to ensure that all information furnished in SSPC guides is as accurate, complete, and useful as possible, SSPC cannot assume responsibility nor incur any obligation resulting from the use of any materials, coatings, or methods described herein, or of the Guide itself.
Painting System Guide No. 8.00 – Guide to Topcoating Zinc-Rich Primers
