Steel ship designers see rust as a reality. Steel yacht designers often must assume that their vessels will not rust at all. The former thicken plates and framing material to allow for a certain percentage of corrosion to take place without jeopardizing the structural integrity of the vessel. Steel sailboat designers, on the other hand, can't afford the extra weight of thicker plate, and when it comes to 10-gauge topsides, decks and cabin houses, there's no tolerance for rust at all.
In theory, a thick, hard-impervious paint coat should protect mild steel forever. In practice, there are no perfect coatings, but some are much, much better than others and every metal boatbuilder, pro as well as amateur, agonizes over just how much of the budget to commit to rust prevention.
Before stirring the paint pot, there is one step that should never be left to chance: surface preparation. Lack of it brings down many a fine steel boat. During the fabrication process, mild steel plate and framing stock develop a surface scale rust that must be removed in order to protect the steel properly from further oxidation.
The best coating approach - flame spraying - is also the most physically demanding. It's a process in which the boat is "white metal" sandblasted in stages, followed by immediately flame spraying hot zinc onto the bright shiny surface. The thick metal coating is virtually soldered, sealing out moisture and oxygen. The zinc also has a good galvanic relationship to steel. This leapfrog blasting and blowing molten zinc onto the metal skin makes grinding out fiberglass blisters seem like a fun sport. Steel boat lovers, there's no better way to extend the relationship.
Chemical companies have come up with some wonderful coatings that can be applied over a zinc-sprayed skin or in lieu of it. The current trend is toward epoxy primers with intermediate and topcoats of linear polyurethane. To get proper protection, manufacturers recommend that certain mil thickness buildups must be achieved, and proper between-coat preparation is a must. Paint manufacturer allegiance is important, and sticking with one vendor's line of products from the first primer coat to the last topcoat will pay off in the long run.
Epoxy primer coatings are near-miracle products with just a few bad habits. For example, if they are applied late in the day, the dropping temperature associated with the onset of nightfall can cause a phenomenon referred to as "amine blush." This haze can impede the adhesion of future coats and should be wet sanded and washed off the surface before another coat is applied. It's also important to remember that, no matter how good a coat of epoxy looks on the topsides, it should never be the final finish. Ultraviolet light attacks epoxy paint, and in a very short time the glistening satin white skin will start to rub off like talcum powder.
Fortunately, epoxy undercoaters love to bond with linear polyurethane paints. A slick, glossy, easy-to-clean finish is quite achievable. However, keep in mind that the glossier the paint the more noticeable any surface imperfections will be. Flattening agents can be added to the topcoat paints to tame down the gloss a bit. It's best not to go too wild with this additive because it does tend to cause the coating to chalk prematurely compared to the same paint with no additive.
Inorganic zinc primers, chlorinated rubber paints and other non-epoxy/LPU alternatives are often used on commercial ships and workboats. They protect well but do not afford the same level of cosmetic finish that can be achieved with LPU topcoats. I prefer spray application of coating materials because large-mil thickness buildups are often required, and by spraying, better control of the material can be achieved. Roughly applying a primer and then sanding half of it off to achieve a smooth surface is counterproductive.
The cost of coating a 40-footer can range from under $1,000 for the bargain basement do-it-yourselfer satisfied with oil-based primer and house enamel, content to slap on a short-lived finish with a hairy roller, to the fastidious extreme that can cost $30,000 or $40,000 for a fair-as-glass automotive finish over a zinc-sprayed white-metal-blasted hull.
Belowdeck, steel must be as well protected as it is on the topsides. Often serious damage is caused by corrosion in the hard-to-get-at nooks and crannies that trap moisture. Before all the trim gets put in place, the interior of the boat should be white-metal blasted and coated. This is an important time to be present in the yard to see that appropriate attention to detail remains in the minds of the blaster and paint crew. For best results, the painter should immediately follow the sandblaster. In situations where blasting is done on Friday and the interior painted Monday, there's a high likelihood of trapping too much scale and moisture beneath the paint film.
I have cut apart many 10- to 20-year-old spade rudders constructed with mild steel cores and found shiny metal once I scraped away the urethane foam. Properly foaming the interior on a steel boat can add both insulation and corrosion protection. Unfortunately, foaming over a rust-scaled surface can hide some serious future problems. Much of what goes on in the coating process revolves around how things are done, not just what is being done.
Longevity depends on how you maintain the boat. If the skin was not zinc sprayed, every scrape and nick will rust if not quickly repaired. Below the waterline, exposed zinc can behave like a sacrificial anode. When exposed, it will galvanically corrode, leaving bare metal to corrode.
Rust is a steel boat owner's toredo worm. Steel is anything but maintenance-free, but by paying attention and following through with touch-up work when necessary, a steel cruising boat can be kept in her prime for decades.
Ralph Naranjo is Cruising World's technical editor.