Urethane Painting Systems
Painting System Guide No. 17.00 – Guide for Selecting Urethane Painting Systems
1. Scope
1.1 This guide outlines urethane painting systems for structural steel surfaces. There are three types of urethane coatings covered by the guide. They are Types II, IV, and V, as classified by ASTM Standard D 16.
COMMENT: One-Package Moisture-Cured Urethane Coating (Type II): This coating contains moisture curing isocyanate terminated prepolymers which cure by reaction with moisture from the atmosphere. Reduced cure rate will result from insufficient atmospheric moisture (low relative humidity).
Two-Package Catalyzed Urethane Coating (Type IV): This coating contains isocyanate terminated prepolymers which cure by reaction with a low molecular weight reactive catalyst. One package contains a prepolymer similar to Type II; the second package contains a relatively small quantity of catalyst to accelerate the room temperature cure. This coating has limited pot life after mixing.
Two-Package Polyisocyanate Polyol-Cured Urethane Coating (Type V): This coating contains isocyanate terminated prepolymers which cure by reaction with hydroxyl-bearing polyols. One package contains the isocyanate terminated prepolymers and the second package contains a large quantity of resinous material containing active hydrogen groups. This coating also has limited pot life after mixing.
1.2 These painting systems are suitable for use on parts or structures exposed in varied types of environments ranging from severely corrosive environments to mild atmospheric conditions.
1.3 These painting systems are intended principally for structural steel where excellent weathering, color retention, and chemical resistance is desired. The color of the finish must be specified.
2. Description
2.1 This guide outlines the components of a complete urethane painting system. A painting system shall consist of surface preparation by commercial blast cleaning or pickling, one coat of a compatible primer, and one or more urethane topcoats.
3. Referenced Standards
3.1 The latest issue, revision, or amendment of the referenced standards in effect on the date of invitation to bid shall govern, unless otherwise specified. Standards marked with an asterisk (*) are referenced only in the Notes, which are not requirements of this specification.
3.2 If there is a conflict between the requirements of any of the cited referenced standards and this specification, the requirements of this specification shall prevail.
3.3 SSPC STANDARDS AND JOINT STANDARDS:
PA 1 Shop, Field, and Maintenance
PA 2 Painting of Steel Measurement of Dry Coating
PA Guide 3 Thickness With Magnetic GagesA Guide to Safety in Paint Application
PA Guide 4 Guide to Maintenance Repainting with Oil Base
SP 2 SP 3 SP 5/NACE No. 1 SP 6/NACE No. 3 SP 8 or Alkyd Painting Systems Hand Tool CleaningPower Tool Cleaning White Metal Blast Cleaning Commercial Blast CleaningPickling
SP 10/NACE No. 2 Near-White Blast Cleaning
3.4 AMERICAN SOCIETY FOR TESTING AND MATERIALS (ASTM) STANDARD:
D 16 Terminology for Paint, Related Coatings, Materials, and Applications
D 3925 Standard Practice for Sampling Liquid Paints and Related Pigmented Coatings
4. Surface Preparation
4.1 SSPC-SP 6, “Commercial Blast Cleaning,” or SSPCSP 8, “Pickling.” If specified in the procurement documents, better degrees of blast cleaning shall be substituted (SSPC-SP 5, “White Metal Blast Cleaning,” or SSPC-SP 10, “Near-White Blast Cleaning”).
COMMENT: For new steel, blast cleaning or pickling of the steel is the minimum recommended surface preparation. Better degrees of blast cleaning (SSPC-SP 5 or 10) may be substituted. These methods are more thorough, and the better cleaning they provide may be more economical or may be required in corrosive conditions. Mill scale is particularly detrimental to the life expectancy of coatings in immersion or wet service.
In maintenance repainting, if blast cleaning is not feasible, hand or power tool cleaning (SSPC- SP 2 or 3) may suffice. These lesser degrees of surface preparation will reduce the service life of the coating system.
5. Paints
5.1 PRIMERS: After cleaning, the steel shall be primed with one coat of compatible primer.
COMMENT: Typical types of compatible primers include organic and inorganic zinc rich, epoxy, universal type copolymer primers, urethane, and wash primers. Primers which can be topcoated with urethane coatings are available for use over lesser degrees of surface preparation (SSPC-SP 2, “Hand Tool Cleaning,” or SSPC-SP 3, “Power Tool Cleaning”). Over such cleaned surfaces these primers will perform less effectively.
5.2 INTERMEDIATE COAT: Where an intermediate coat is required, it must be compatible with both the prime coat and the finish coat, but preferably in a contrasting color. Other generic intermediate coats as recommended by the coating supplier can also be used.
5.3 URETHANE TOPCOATS: Aliphatic and aromatic urethane topcoats can be used.
COMMENT: Aliphatic urethane topcoats are recommended where the highest degree of gloss and color retention, along with chemical resistance and recoatability, are desired.
Aromatic urethane topcoats are recommended where gloss and color retention are not required, but where chemical and abrasion resistance are desired. Aromatic urethanes require mechanical abrasion prior to recoating.
Typical properties of these urethane topcoat types are shown in Table 1.
6. Paint Application
6.1 PAINT APPLICATION: Follow requirements of SSPCPA 1, “Shop, Field, and Maintenance Painting of Steel.”
COMMENT: The primer and all subsequent coats must be compatible. Therefore, before using urethane coatings on previously coated surfaces other than those recommended, compatibility tests should be made for possible lifting or intercoat adhesion problems. Aliphatic urethanes normally have good recoatability while aromatic urethanes require mechanical abrasion prior to recoating.
Application may be by brush, roller, air spray, airless spray, electrostatic spray, or a combination of these meth ods. Dipping, flow coating, or hot spray is permitted only when specifically authorized by the coating supplier.
Some urethane coatings cure at temperatures as low as -7°C (20°F) and some can be applied as thick as 150 to 200 micrometers (6 to 8 mils) dry film thickness in one application. Drying times and minimum elapsed time permissible between coats of urethane coatings may vary with type and coating supplier. Particular attention should be paid to these points. Usually, it is good practice to obtain the complete coating system from one supplier.
Consult the coating supplier for special handling, industrial hygiene, and storage instructions.
6.2 FIELD TOUCH-UP PAINTING: In accordance with specification SSPC-PA 1, “Shop, Field, and Maintenance Painting of Steel” and in particular with the Section thereof entitled “Field Painting.”
6.3 MAINTENANCE PAINTING: For maintenance painting procedures, see SSPC-PA Guide 4, “Guide to Maintenance Repainting with Oil Base or Alkyd Painting Systems.”
COMMENT: This guide covers the steps necessary for repainting previously painted steel surfaces.
6.4 NUMBER OF COATS: A minimum of two.
COMMENT: The number of coats will depend upon the service environment, and the recommendations of the coating supplier should be followed.
6.5 DRY FILM THICKNESS OF PAINT SYSTEM: Film thickness shall be measured in accordance with SSPC- PA 2, “Measurement of Dry Coating Thickness with Magnetic Gages.”
COMMENT: The recommended dry film thickness of each coat of the complete paint system may vary with the end use and metal surface preparation. Prior to application, the desired thickness of each coat should be agreed upon. The recommendations of the supplier should be followed.
7. Inspection
7.1 All work and materials supplied under this specification is subject to timely inspection by the purchaser or his authorized representative. The contractor shall correct such work or replace such material as is found defective under this specification (See Note 10.1.). In case of dispute, unless otherwise specified, the arbitration or settlement procedure established in the procurement documents 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 shall be supplied upon request along with the manufacturerʼs name and identification for the materials. Samples may be requested at the time the purchase order is placed, or may be taken from unopened containers at the job site.
7.3 Unless otherwise specified, the sampling shall be in accordance with ASTM D 3925.
Painting System Guide No. 17.00- Guide for Selecting Urethane Painting Systems
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