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A coatings challenge

Jan 13, 2010

Material selection in the marine environment is a tricky business, and when it comes to sailboat keels there’s more at play than the simple quest for cost-effective ballast. Iron and steel work well when it comes to strength, but they have obvious corrosion problems in salt water. Happily, improvements in epoxy coatings have a viable option of the use of iron and steel.

Many modern efficient racer/cruisers have been moving more and more toward the performance side of the fence, and in doing so their keels have begun to take on a distinct new look. Nearly gone are the good old encapsulated ballast keels that did away with keel bolts by placing the lead in a long, wide, fiber-reinforced plastic keel cavity. Almost as antiquated are the lengthy, somewhat foil shaped external lead ballast keels that were bolted to deep sump-like keel stubs. Both of these options have some strong selling points, one of which is their willingness to stay attached to the hull and their reduced corrosion problems.

But more performance-oriented racers and cruisers have rallied around deep, narrow, high-aspect-ratio, foil-shaped appendages that provide more lift and less drag. When cast in lead and bolted directly to the hull, these thin, short chord length keels have a tendency to bend even in minor grounding situations, despite the fact that they had been alloyed with an appropriate amount of antimony.

One solution to the problem comes from the thoroughbred part of the racing world where the fin and bulb concept has been carried to even further extremes. At first, America’s Cup boat designers and others tried a composite approach, centered on an elaborate carbon fiber/epoxy foil used to support hefty lead ballast bulbs. These appendages were often a dozen feet or so in length and were flex, with no worry over breaking strength that dissatisfied crews. Fortunately, a few well-schooled engineers knew that there were metal alloys with isotropic properties that would deliver much less keel sag for the same given area and cross section. These were often high-carbon ferrous metals that brought with them serious oxidation issues.

Corrosion abatement
This solution, however, became a coatings challenge, and as more and more builders opted for a lead bulb, steel foil and stainless steel keel bolts or stud fasteners, corrosion abatement moved center stage. The Europeans were quick to lock onto the new keel concept partially because iron keels had long been de rigueur in their part of the world. They put up with the issue of rust, now greatly diminished by better coating options.

With lead costs skyrocketing, many production builders are reverting to iron ballast even when a shoal draft keel would be much more efficient if made from lead.

Fortunately, owners of boats with iron appendages, whether they are driven by a designer quest for performance or a builder’s parsimony, have an ally in paint technology that can help keep rust under control. At first the challenge seems simple, just keep sea water and its mischievous chloride ion away from the ferrous metal and you’re home free. The task is like taking a handful of steel washers, putting them in a zip-close bag and tossing them in the drink for the season. Pull out the bag, and providing that there were no leaks, the washers will be good as new. The big question however, is what’s the sequel to a zip-close bag for a six-foot-long steel keel fin? Epoxy is the answer, and numerous coating manufacturers provide a highly adhesive, waterproof system with which to barrier coat submerged iron. But it’s the skill of the applicator and the preparation of the surface that, in the end, make or break a coating’s effectiveness.

Like most problems that are solved with simple solutions, some of the answers are too good to be completely true. And despite the coatings break through, keeping ferrous metal from corroding in seawater is anything but a boatyard sequel to stare decisis (settled law). The reason for this is rooted in the chemistry and physics most of us daydreamed through in high school. Things like water being the universal solvent, the tenacity of galvanic corrosion and the woe of stray current electrolysis’ all team up to shake off well-adhered coatings. To add insult to injury, keel flex, submersion pressure, seasonal heating, cooling and freezing, plus surface abrasion from scores of unexpected contacts all team up to assault epoxy coatings. Fortunately, iron corrodes a little slower than high carbon content steel and there’s ample warning when it’s time to tackle a spot repair. But if ignored, the time will come when the entire keel surface must be refinished and all paint, putty and primer must be painstakingly removed. The process is a daunting roll-up-your-sleeves, or open-up-your-wallet experience.

Step-by-step Paint removal is no one’s idea of fun at the beach, and in the case of repairing a badly rusting ferrous metal keel the quality of the surface prep seals the deal on finish longevity. In fact, the process of removing paint and pitted, flaking rust from a ferrous metal surface is one of the toughest challenges a DIYer can face. The best answer is to have the metal sand blasted by a pro — a process that removes the surface coatings and oxidized metal. Lurking in the sand blaster’s wake is a shiny but short lived silver metallic luster and an even shorter window of opportunity for the paint crew or DIYer to react. If sand blasting cannot be carried out, be prepared for a test of patience and tenacity as you scrape, grind, wire brush and cajole a shiny surface out of the rust flaking, pitted iron keel.

The most finicky of coating experts know that timing is everything, and that they must be ready to quickly follow in the footsteps of the sand blaster. The new shiny surface starts to oxidize the moment it is exposed. This means that the best results are derived by immediately air blasting to remove grit and residue. Next comes a solvent wipe down with acetone or an MEK reducer to eliminate any oily spatter left on the surface thanks to leaky seals or worn rings in the compressor. Finally an acid etch/primer tie coat is applied as thinly as possible. These acid/chromate primers have very poor shear strength and are meant to act as a chemical etching treatment rather than as a tenacious bond producing barrier. The latter is left to the epoxy coating system that follows, and it should be applied with enough physical force so that porous metal surfaces are completely coated without inadvertently creating any entrapped air bubbles. Amateurs using a foam roller, brush and plastic squeegee can do a good job if they “work” the material into porous areas. Pros often use an airless pressure sprayer to achieve the same result. Attention to detail and careful squeegee use is necessary to force epoxy primer into deep fissures and tiny recesses where pitting has been removed.

Paint companies like Interlux, Pettit and Awlgrip offer a product line with specific coatings and step-by-step instructions for refinishing submerged ferrous metal surfaces, go to their Web sites for specific details. In each of these systems the importance of surface prep is highlighted and all barrier coat protection is based upon the use of a high solids epoxy coating that seals the surface both prior to, and after epoxy putty fairing has been carried out.

The need for complete resurfacing is contingent upon how badly the keel has deteriorated. Keeping up with seasonal spot repairs can postpone the inevitable. But when a good bit of the keel starts to look like an astronaut’s view of the lunar surface, the time has come to tackle a complete make over. It may take several fill and sand repetitions to turn the moon’s profile back into a smooth contour, but even more important is what precedes the fairing process. If rusted metal is covered over, there’s a good chance that the coating will fail sooner rather than later. Sealing bright, shiny, clean metal under an etch primer and epoxy barrier coat prior to fairing with epoxy putty will pay off in the long run. Stick with one paint manufacturer’s lineup of products, be ruthless in riddding rust and don’t skimp on the epoxy primer. Better yet, get an estimate from your boatyard on a keel redo and then consider whether lead ballast makes sense after all.

Ralph Naranjo is a writer and photographer based in Annapolis. He is the author of the book Boatyards and Marinas.
 

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