COLOR

This subject will probably be the first thing that comes to mind when you, or at least the family, are considering the paint job for your new or used metal boat. Even a simple matter like choosing the colors has its technical side. In my opinion darker hulls look better than white hulls. This may be especially true in powerboats. To my eye a dark blue hull adds "class" to any boat. That's just my opinion, but it's something to consider as you look at various other boats to help you decide on a colour scheme for your metal boat.

   In metal boats, a darker colour for the hull also makes good sense. The darker colour will absorb sunlight and drive off both the dew and some internal condensation. If the decks are painted, then you may choose a two-tone scheme of light beige for the larger areas and cream for the trim or unsanded areas. This arrangement will look smart and it will be easy on the eyes. You should never paint decks white, as the reflected light will cause too much glare. Except in the coolest climate, dark-collared decks will be too hot for bare feet and will also make the interior of the cabin unbearably hot.

GRIT-BLASTING AND PRIMING STEEL

If your boat was constructed from raw steel straight from the mill you will need to grit-blast the metal before attempting to apply any paint coating. This process is variously referred to as sandblasting and grit-blasting. It’s the only way you can provide a satisfactory base on which to apply your prime coating and subsequent layers of protective paint. Blasting is necessary to remove all contaminants and corrosion from the surface of the metal; and as this process slightly roughens the surface, it also provides an excellent “tooth” for the paint. For reasons that will become clear, we believe the best way to build a steel boat is to use pre-shotblasted and primed material, and to build under some form of cover. Nonetheless, some builders do blast their own steel.

   The exact roughness of the surface will depend, for the most part, on the particular metal, the type and grade of grit used, and the force with which it’s applied during the blasting process. The result is commonly referred to as the anchor pattern, and will vary between 1.5 mils (thousandths of one inch) and 4 mils (0.038 mm and 0.1 mm) in depth. Four thousandths (0.1 mm) is considered a heavy and deep blast, and may be satisfactory for tar-epoxy finishes. For most paints, however, you should aim for a 1 1/2-mil to 3-mil (0.038 to 0.076 mm) anchor pattern. Your paint manufacturer may have special recommendations in this area. Make sure you choose a warm and dry day with low humidity when you are blasting your boat.

   For blasting, you’ll need a powerful compressor; for instance, one that can deliver something over 350 feet (107 m) per minute would be perfect. You need manpower: two or three extra people should be adequate. There are various specifications for the blasting of steel. ear white blast-cleaning will deliver a successful, corrosion-free, and long-lasting paint job. Near white blast-cleaning produces a surface in steel that, when viewed without magnification, is free of all visible oil, grease, dirt, dust, mill scale, rust, and paint. Generally, evenly dispersed, very light shadows, streaks, and discoloration caused by stain of rust, stains of mill scale, and stains of previously applied paints may remain on no more than 5 percent of the surface.

   Sand is the least expensive abrasive, but because of its high silica content and the health hazard presented by silica, you may find that your local contractor is not willing to use this material. If you’re doing the job yourself, and you wear the correct protective face mask, then blasting one boat with this material should not represent an undue health hazard. It’s your responsibility to decide on using sand as opposed to the more expensive alternatives. Sand can’t be reused, so you’d need more sand than slag or grit.

Grit is a little more expensive than sand but contains none of the silica that, when used over a period, can cause respiratory and lung problems, among other things. In the United States, these products are marketed as Copper Blast, Copper Slag, Green Diamond and Garnet. Similar products are available in Britain, Australia, and elsewhere.

   Crushed-steel shot is more expensive than either sand or grit, but as it can be reused many times you may consider it to be worth the additional expense. It’s formed from crushed iron or steel, and has irregular shapes with very sharp edges. It’s one of the better blasting materials.

   You’ll need to consider matching the type and roughness of the blasted surface with the paint you plan to apply. A surface that’s too rough will show through your paint finish, and a surface that’s too smooth won’t provide a good grip for that most important element in your paint job, the primer.

   Many builders have found that it takes a lot of effort to undertake this job, and it saves very little in cost. By the time you hire the equipment, purchase the grit, and arrange for the help needed, it can cost nearly as much as a professional job. You need three people for this operation; one to operate the blasting gun, one to feed the material, and one to apply the primer. If you don’t have three people, the job will take much longer. For instance, if the blaster has to feed his own abrasive, he has to remove and replace his bulky helmet each time more grit is needed: a time-wasting exercise.

In the case of steel hulls, the prime coating must be applied immediately after the blasting. This means that the painter has to follow the blaster as closely as is practical. Rust can form in a surprisingly short time; the actual time depends on the weather and humidity prevailing when the blasting takes place. It’s usual for the owner or his employee, as opposed to one of the contractor’s employees, to undertake the painting. If you control this critical job yourself, you’ll be assured of its success. Make sure the paint is being applied to a perfectly clean and dry surface at a minimum temperature of 50°F (10°C). The paint is best applied within 30 minutes of the blasting. That way, as well as avoiding rust, you’ll have the extra advantage that the steel should still be warm from the blasting. Finally, 3 to 4 hours is the absolute maximum time lag between blasting and prime coating. This is definitely not a one-person operation.

   Estimating the time it takes to blast and prime coat is difficult, given all of the variables involved. However, about 50 square feet (4.65 square meters) per hour seems about average. No grit operator or painter can operate flat-out for extended periods. It’s wise to divide the hull, deck, and superstructure into reasonably sized segments; say, quarters on a small- to medium-sized hull, and smaller proportions on larger craft. Keeping the blasting and the painting apart will take some organizing, but you will need to do it to ensure a clean and long-lasting priming job.

   When you purchase pre-blasted and primed steel from a specialist supplier, then these materials have most likely been blasted by a wheel rather than blasted in the regular way. This process is most effective in plate of 1/8-inch (3 mm), or 10-gauge, and larger thicknesses, but lighter plates may distort when exposed to wheel “blasting.”

Wet blasting involves using water mixed with the grit or sand. This process keeps down dust, but afterward it leaves a great deal of heavy, wet grit or sand to be cleaned up. A rust inhibitor is used in the water, but you have to blow the hull dry and apply the paint before the effects of the inhibitor disappear.

   The interior of the hull, under the decks, and inside the superstructure will all need to be grit-blasted: a difficult and messy job. Don’t forget to have all of the cut-outs and openings for windows, ports, and hatches already completed before you grit blast and apply the prime coat. It will probably be best to blast the outside first; that way, you won’t have any worry about grit coming through openings into an already prime-coated hull interior.

   We have now come full circle. We’ve considered the alternatives and you can see that my advice to use pre-shot-blasted and primed material makes sense. Even if you are building in the open, depending on the climate and the amount of care you take in covering the hull, you may find that the pre-blasted and primed materials are worth the extra cost and effort. Even so, covering your hull during non-work periods will pay off.

If you use pre-prime-coated steel, then you’ll need to clean up only in the area of the welds. You’ll recoat these areas before proceeding with another prime coat of the entire hull, deck, and superstructure. The welds can be cleaned with a grinder, a wire brush, or a similar device to expose a clean surface that’s ready to be touched up with matching prime coat.

   Be careful when you’re using solvents and other liquids to clean metal; be aware of the deposits they leave behind, so that you’re not faced with a never-ending circle of cleaning and re-cleaning a particular area. Acids can sometimes be used to advantage to remove surface contaminates, including rust. Acids tend to etch the surface and thus improve the adhesion of the paints you apply afterward. Generally, acids are only used as cleaning agents in smaller areas such as those where welding has spoilt an otherwise prepared and primed hull.

Filling and Fairing

   For a perfect or near-perfect finish, almost all metal boats need some filling and fairing, but it’s important that you do not rely on the filling compound to cover sloppy workmanship. The following advice will probably be ignored by the sloppy builder and resented as unnecessary by the perfectionist. It’s to the greater proportion of you occupying the middle ground that we address this advice. Your aim should be to make every step of the building process produce a fair and smooth hull, deck, and superstructure. You should strive to build a boat that will require the minimum of filler. If you are building from a pre-cut kit you will find that very little filler is required. Pre-cut kits do make for a fairer hull than can ever be achieved by building from scratch. If your hull is truly fair you will only need to apply filler in the areas where the plate is joined or where welding and grinding has taken place. Often just a smear of filler will do the job.

   Now, having established that all hulls and most superstructures need at least some filler to produce the near-perfect appearance, it’s simply a matter of choosing and correctly applying the right material. Automotive body putty is not the correct filler for your boat. It won’t withstand the rigors of marine use.

   Incidentally, please let the recommendations of your paint manufacturer overrule any advice we give here. You must choose one manufacturer and use its products exclusively. If you mix brands, you’ll have no protection if the product fails. Each manufacturer will blame the competitor’s product as the cause of the problem.

The correct filler for your metal boat should be epoxy, not polyester based (as is usual) with automotive fillers. Your fairing compound should contain inert fillers such as micro balloons. Many paint manufacturers have their own fairing compounds as part of the overall paint system. Make sure you choose a manufacturer who will give you local advice and technical assistance. This is not just a case of visiting your local marine store and taking what’s on offer. You’ll need to undertake considerable research to ensure you end up with a long-lasting and attractive paint finish on your metal boat. To quote our own experience, our 38-foot (11.58 m) steel powerboat was originally painted in 1991, and today the superstructure looks as good as new. Due to mishandling and neglect by the previous owner, the hull recently needed a blow-coat to cover scuff marks.

SPRAYING HOT METAL

This method of applying a protective coating is included here because we’re still occasionally asked about its merits for a steel hull. Metal spraying was at one time popular with some steel builders. During the 1970s, when it was most popular, there were many who decried its use on the grounds that if it chipped or otherwise failed, water would creep underneath and cause considerable invisible corrosion problems. Time has proven these critics correct, in fact, and the method is infrequently used today. Another drawback was that the materials used for these coatings were notoriously averse to holding paints as intended. The development of modern epoxy and urethane protective coatings has allowed flame spraying (or metalizing, as it was popularly known) to fade from the scene. In the interests of thoroughness, however, here are the details.

   Hot-metal spraying is accomplished by melting either zinc or aluminum metal wire in a special gun that drives it at high speed onto the bare steel in the form of molten droplets. Like a surface prepared for painting, the steel surface must be prepared by grit-blasting down to white metal. Without this etched surface, the hot metal spray will either roll off or flake off after cooling.

   Many advocates of this method claim a chemical bond forms between the aluminum or zinc and the steel; they claim that the metals are fused together. Actually, the hot metal spray forms a mechanical bond only. It depends on the correct spraying techniques, as well as a grit-blasted surface, to maintain its grip on the steel. If you plan to metal-spray your hull, don’t use sand as the blasting agent. Commercially manufactured grit is necessary to give the correct key for the metallization process.

If you use a hot spray involving aluminum or zinc, then you should apply a special wash to the aluminum coating before applying any paints. An example of such a wash is Interlux Viny-Lux Prime wash, which is specially formulated to adhere to bare aluminum and is a good primer for the other coats that will follow. If you are considering one of the hot metal spays for your boat, you should seek out the latest information on the subject. My advice is stick to the well-proven regular painting procedures for metal hulls.

PAINTING

In the process of selecting the paint, you’ll have to consider how it will be applied. Certain finishes lend themselves better to one application method than another. Some paints can be applied in many ways, so this may influence your choice of paint as well as what equipment you either purchase or hire for the job. No matter what method you choose for applying the various paints, you’ll need a selection of brushes, rollers, paint trays, scrapers, sandpaper, and all the usual tools one associates with painting any structure. We’ve seen many fine metal hulls painted with hand tools, including a combination of rolled and brushed finishes.

   Airless, or air-assisted spray equipment is favoured by those experienced in painting hulls, and it’s possible to lay on high-build paints in a way that could not be achieved by hand. If you elect to blast your boat, it’s well worth considering using a professional team to at least apply the prime coating on your hull immediately after blasting. A team of, say, three or four professionals can blast and prime your boat in just a few hours and thereby ensure that you get the best cover for the blasted steel before it starts to rust.

The success of your paint job will depend on the care and attention you lavish on the preparation of your vessel before the first finish coat is applied. You must identify individual items that will be more prone to rust, and then give them additional attention. One way to identify potential problem areas is by studying other older steel boats. If you’ve followed our advice on layout and construction, then you will already have avoided most of these potential problems. Now, all you have to do is carefully check your own boat before you start to paint.

   Usually, rust doesn’t form on smooth areas of the hull. Irregular and sharp surfaces are often the culprits. For instance, we have always recommended that you avoid sharp corners on your hull or superstructure. To avoid creating rust traps and areas where paint is easily damaged, liberal use should be made of split pipe and/or rolled plate. When you eliminate the potential problem areas, you will also eliminate the corrosion problems that, at best, ruin the appearance of any metal boat, and at worst endanger its security. Therefore, no sharp edges, sharp corners, water traps, overlapping plates, or other bad practices that we have already covered in earlier chapters.

   Don’t attempt to paint areas of high wear, such as anchor fairleads, cleats, and similar fittings. They must have a stainless steel liner welded in place to accept the wear, and thus avoid any corrosion problems. All welds in areas above the waterline must be ground smooth and filled. This will ensure that no jagged edges or high spots are present. Sharp corners and jagged welds prevent the layers of paint from covering evenly, and thereby diminish protection.

   As we’ve said already, we don’t recommend that you grind smooth the welds below the waterline. This is a safety factor, and it means that these welds should be most carefully executed to give maximum strength and maximum smoothness, to enable them to accept a full quota of paint.

ALUMINUM

Many aluminum workboats are left unpainted and this is not a problem when the correct grade of marine aluminum has been used to build the vessel. The metal forms an oxide on the surface and further protection is unnecessary. But even these unpainted aluminum vessels need some protection below the waterline, so the underwater hull must be coated with a suitable antifouling paint. In France, we’ve seen many aluminum-hulled sailboats and powerboats with unpainted topsides, and quite frankly, they look unfinished. Unless you want your boat to look like an untidy workboat, you should accept the fact that you will be painting your aluminum vessel.

  You’ll need to abrade the surface of your aluminum hull by sanding it, or by using abrasive pads to roughen the surface of the metal. Next, thoroughly clean the surface with the chemical preparation recommended by your particular paint manufacturer. Now etch the surface. This is usually done with a phosphoric acid solution that changes the chemical properties of the surface of the aluminum, allowing better adhesion for the first coat of paint. Your paint manufacturer will recommend an etch primer or a wash. A primer coat will follow this. Needless to say, this primer coat is one of the most important of the whole system; if it fails, then the whole system will break down.