Like other topside areas, the masts and stacks are exposed to the harsh environment of salt air, sea spray, and constant exposure to the sun’s UV rays. In addition to these, masts and stacks are also exposed to exhaust gases. On DDG 51-class ships, the tops of stacks are also considered critical coated areas.
Typical Coatings Used
These surfaces are coated with an anti-corrosive primer and top-coated with haze gray paint. Coating systems in these areas must be able to withstand the harsh environment while also retaining their color and gloss.
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. For masts and stacks exposed to gases, acceptable coatings include MIL-PRF-23236, Types V, VI, VII; Class 5 or 7. Type VIIIA coatings may be used on powder coated watertight doors and bolted louvers serving mast and stack areas, though they may be applied by fluidized bed method only.
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 masts and stacks.
The topcoat must be qualified to the performance specification MIL-PRF-24635, Coating Systems, Weather-Resistant, Exterior Use.
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. For steel substrates, 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. For aluminum substrates, NSI 009-32 requires Near-White Metal Blast using garnet, aluminum oxide, crushed glass, walnut shells, or stainless steel shot for abrasive blast media; or SSPC-SP WJ-2/NACE WJ-2, Very Thorough Waterjetting.
The surface metal must be degreased following abrasive blast with walnut shell media. Any traces of residual oil will degrade paint adhesion. Appropriate safety precautions for working with flammable solvents must be enforced. Alternatively, a vigorous soap and water wash followed by pressurized fresh water rinse may be used. Do not use a detergent and fresh water washdown when using aluminum oxide abrasive blast media.
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 Repairs
Ship’s force personnel are primarily responsible for maintaining the ship’s stack and mast 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
New construction ships are painted in accordance with the ship build specification for that class of ship.