FREEBOARD AND SUPERSTRUCTURE COATING SYSTEMS
The freeboard is the area of the hull above the waterline up to the main deck. The superstructure is the ship’s vertical structure that rises above the main deck. These exterior surface areas are subjected to corrosive sea spray and ultraviolet radiation.
This large, very visible area is coated with an anticorrosive coating system followed by a topcoat of “haze gray” paint for most surface ships. The anticorrosive paint prevents material degradation and the topcoat protects the surfaces against ultraviolet radiation. In addition, the topcoat may contain low-solar absorbency and anti-stain pigments to reduce the amount of heat absorbed by the ship and help prevent rust staining.
The primer, stripe coat (if applicable), and intermediate coats are anticorrosive coatings that are qualified to either MIL-PRF-23236, Coating Systems for Ship Structures or MIL-PRF-24647, Paint System, Anticorrosive and Antifouling, Ship Hull.
The MIL-PRF-23236 specification divides the coating system into four types based on the volatile organic compound (VOC) content. This specification has nine classes of coatings that are determined by the type of surface being preserved. Class 5, 7, or 18 coatings are used for exterior structures.
As an alternative to the primer, stripe coat, and intermediate coat method, NAVSEA Standard item (NSI) 009-32 recommends a single coat system using MIL-PRF-23236 Type VII, Class 5/18 or 7/18 paint.
NSI 009-32 specifies two types of anticorrosive coatings qualified to the specification MIL-PRF-24647 for the primer, stripe, and intermediate coats. MIL-PRF-24647 Type I or Type II coatings (i.e., those that have topcoats that may or may not contain copper biocides which ablate or self-polish) are authorized for use on the freeboard and superstructure.
The topcoat must be qualified to the performance specification MIL-PRF-24635, Coating Systems, Weather-Resistant, Exterior Use. A MIL-PRF-24635 coating system is either a polysiloxane or a silicone alkyd. The MIL-PRF-24635 specification divides the coatings into five types based on VOC content and whether it is a standard or high-durability coating. This specification defines three classes of coatings: high gloss, semi-gloss, and low gloss. The grade of the coating is determined by the pigment. Those grades are standard pigmented, low solar absorbent, and low solar absorbent and anti-stain.
Low solar absorption (LSA) coatings are designed to reduce the heat load and infrared signature of the ship. Interior compartments with one or more bulkheads forming a section of the superstructure stay cooler during warm weather, which benefits both sailors and equipment. LSA topcoats contain additional pigments that increase reflectance without sacrificing color or gloss.
Anti-stain coatings are designed to reduce running rust on topside shipboard surfaces by using chelating agents to turn the rust into a transparent film. This improves the overall topside appearance of the ship, provides an easy to clean surface, and reduces maintenance efforts required by ship’s force.
To ensure proper coating adhesion, special attention should be paid to the surface preparation of the freeboard and superstructure. NSI 009-32 requires a surface that meets one of the following cleanliness standards: SSPC-SP 10/NACE 2, Near-White Metal Blast standard or SSPC-SP WJ-2/NACE WJ-2, Very Thorough Waterjetting.
NSI 009-32 is the governing document for preservation requirements. Permitted coatings are specified in Table 2. All freeboard and superstructure coating system preservation efforts must be accomplished in accordance with NSI 009-32.
Ship’s force personnel are primarily responsible for maintaining the ship’s freeboard and superstructure coating systems until repair activity maintenance is scheduled. Naval Ships Technical Manual: Chapter 631 (NSTM 631), Preservation of Ships in Service – General provides requirements, instructions, and guidance for surface preparation and coating application.
New construction ships are painted in accordance with the ship build specification for that class of ship.