Aaron Porter Painters should wear personal protective equipment when applying most antifouling paints.
Boats demand constant maintenance, but one of the most persistent predictable needs, even for boats that never leave the dock, is the prevention and removal of biological growth on the bottom. Any part of the boat in constant contact with the water the boat floats in is susceptible to marine growth. Depending on the water’s warmth, salinity, and nutrient loading, that growth can range from a light algae layer on a biological film of microorganisms to heavy biofouling of marine plants and animals like mussels and barnacles that feed on plankton in the water. Their accumulation can have all kinds of detrimental effects on a boat—reduced speeds, excessive fuel consumption, or in the case of the dreaded shipworm, catastrophic structural failures. Any surface not coated in a protective paint with antifouling properties requires frequent scrubbing to prevent unsightly stains and biological growth.
In most all waters, manual removal is impractical, so seafarers have treated hull bottoms for many hundreds of years with a range of coatings from tar, tallow, and hot pepper to mercury, arsenic, and tin paints, with varying degrees of success. One of the more effective antifouling coatings discovered was copper, applied in the form of thin sheets affixed to a ship’s bottom or as powder mixed with paint. (For more on antifouling see Steve D’Antonio’s “Antifouling Tactics” on ProBoat.com.)
Enter the modern era of antifouling coatings. Meant to be efficient and relatively economical, bottom painting has become one of the most dreaded annual maintenance rituals, sometimes requiring bi- or tri-annual replenishment depending on the coating, environment, and vessel use.
This article is intended as a practical guide to this essential maintenance task for professional boatyard technicians and any fastidious or parsimonious boatowners who prefer to paint their own bottoms.
Preparation
Let’s start at the beginning, with forethought and essential planning, when all the important decisions are made. We want to focus on the who, where, what, when, and how of antifouling.
The first of these calls for some self-awareness about your experience level. As a seasoned professional, you may have developed some complacency and think you can skip certain steps. As a new employee or boat owner, you may not even know what steps you are at risk of skipping. In either case, be honest with yourself about your knowledge and abilities as you plan the project.
You should know your yard’s policies, as well as relevant state and local regulations. If you don’t, look them up and remember that location matters whether it is your back yard or your boatyard. Some states may require the painter to have a pesticide applicator’s license, some locales don’t allow sanding of antifouling paints due to environmental regulations, and in others there are special containment requirements should the hull need sanding or stripping below the waterline. Be familiar with these requirements before getting started as an EPA fine for violations can be significant. Keep in mind that removal of old paint may not be required if the boat was coated with an ablative that wears off with use, preventing buildup of old paint.
Aaron Porter A coat or two of copper-based bottom paint is a standard annual maintenance measure, especially on traditional wood hulls.
Indeed, the type of paint chosen dictates much of the process. The best antifouling option is dictated by a boat’s use profile, location, and hull material. That means doing some research. Perhaps you have a houseboat that lives at the dock in the brackish tidal waters of a river delta, and you only get under way once a month. Or maybe you own a charter boat that takes people out fishing daily. Remember the ablative paint? It’s not ideal for the sedentary houseboat but would likely perform well on the charter boat. Consult manufacturer’s recommendations and consider any client preferences as you choose the paint.
The following variables should factor into your choice:
- Type of use: Is the boat sitting relatively still most or all year, is it a race boat, does it live on a trailer year round, or is it docked in a marina or at a mooring?
- Type of water: Salt, brackish, or fresh?
- Hull material: Fiberglass, wood, steel, aluminum, or exotic composites like carbon fiber and Kevlar.
It’s a practical reality that often your paint choice depends on what was selected years before when the first antifouling coat was applied. The standard yard task, barring a change in boat owner or location, is to apply the same bottom paint that has been used on the boat before. If switching paint brands or systems, be sure the new coating is compatible with the underlying residual paint, or that the surface is prepped and primed to accept the new chemistry.
Before making your final antifouling selection, consult the coating manufacturer’s published Technical Data Sheet (TDS) for the product, which includes details such as time to cure before launch, solvents and thinners, pre-application prep, and optimal atmospheric conditions. It’s important to know whether your coating must be immersed in water after two days of cure time or if it can sit two months before launch day. If the paint requires immersion within a set period, this is the time to reserve the Travelift to make certain of the launch schedule.
The TDS will also tell you how to prep and prime the surface for painting depending on whether the hull is bare or has some coatings applied already. In the latter case, it’s important to determine what the previous coatings are to check paint compatibility. Most coatings manufacturers have great tech support, so give them a call to confirm compatibility if you have any doubt.
One last point to consider before starting the real work is personal protective equipment (PPE). Remember, most of this paint is toxic; even the most “environmentally friendly” types of antifouling coatings can cause health issues. Especially for professionals who are exposed daily, the health risks are real. Consult the product’s Material Safety Data Sheet (MSD, MSDS) and follow the listed recommendations. This information will be found on the manufacturer’s website for your product of choice. In general, disposable protective suits, organic compound respirators, nitrile gloves, and chemical-rated goggles should be worn when working with this stuff.
Eric Renshaw The essential tools and equipment for applying antifouling paint are neither expensive nor exotic, but it’s good practice to be sure you have them all at hand before starting the job.
Tools & Materials
Let’s look at a basic equipment/materials list, which will need to be adjusted depending on your specific situation and plans. Make sure you have everything on your completed list handy before starting the job.
1. The antifouling coating appropriate to your project boat, along with the recommended solvents and primers.
How much? Everyone has their own, but this is my formula:
0.85 (X)(LOA) x 2
where X = the distance from the keel to the widest point at the chine in feet, and LOA = the length overall of the vessel in feet.
This gives you a fair estimate of the square footage of surface to be painted. The TDS will give you the square foot coverage per unit the material is sold by (quart, liter, gallon, etc.). For deep fin-keel boats, calculate the keel separately as a rectangle (average width x height) x 2 for both sides.
2. Surface prep materials, per the TDS.
3. Assorted tape—strapping tape for adhesion test, painter’s, and fine-line tape for masking off the waterline (see Jim Moores’s “Tape Technique” Task Sheet in Professional BoatBuilder No. 210, page 54).
4. Plastic or canvas tarps for containing dust, debris, and paint drips.
5. Stiff scraper (and a file to sharpen it) and a heat gun if the paint needs to be stripped.
6. A water supply and garden hose with spray head or a pressure washer.
7. Appropriate brushes, roller covers, and frames, per the TDS.
8. An abundance of paint rags, paint mixer, mixing cups, and roller pans.
Procedure
Surface preparation is the most important part of the job. If the hull has old paint on it, start with an adhesion test; if the old paint doesn’t stay on the boat, neither will the new. To do this, select an unobtrusive area or an area that looks doubtful and mark off a 3″ x 3″ (75 x 75mm) area with pencil. Then score it carefully in 1″ (25mm) squares through the paint but not into the hull itself. Cover the area marked with the strapping tape, rubbing with firm pressure for best adhesion. Peel off the tape, pulling 90° to the surface. If the paint comes off with the tape in layers or down to the hull, it is time to strip.
Aaron Porter Be sure the boat is correctly blocked and supported before getting under it to prep the bottom. Remember, you will need to move the stands to paint the entire surface.
Before you start serious surface prep, make sure the boat is firmly blocked and stable on solid ground. (Note that blocking and stands will need to be moved and the paint patched where they were for the initial coating.) Don’t work on any vessel while it hangs in slings. Trailer boats should be removed from the trailer.
There are many ways to strip a bottom but the most efficient is to sandblast with a corn cob or walnut shell medium. Corn cob is softer and works great on wood hulls. It has the added benefit of revealing soft planks that should be replaced. I used to subcontract the paint stripping out to a mobile sandblaster who dealt with all the mess.
For manual stripping, first check the bootstripe or waterline for paint adhesion issues, and tape it off with a good painter’s tape in preparation for manual scraping below. Suit up in the appropriate PPE, then I go to a heat gun and scraper on wood hulls. This method is surprisingly fast, but I don’t recommend it for FRP/GRP or wood-composite hulls where the heat can damage the resin in the structural hull laminate. For those composite hulls, the solution is insane amounts of sanding. Wet-sanding here is nice as it keeps the dust under control, but it requires a filtered floor drain to catch the debris. (You may want to add cheap rain gear to your materials list should you choose that route.)
Finish sanding the hull surface in the grits recommended in the TDS for the first coating to be applied, be it primer, barrier coat, or antifouling.
Mercifully, in most cases, stripping a hull isn’t necessary, especially when a boat is hauled at least annually and thoroughly cleaned using a power washer that knocks off biofouling and much of the loose paint. So, if you passed the adhesion test, life is good, and all you need to do is prep the hull according to the TDS, making sure there are no hard edges or chips in the remaining bottom paint for water to get a hold of under way.
Some paints require a light scuff with a specific grit. I like to do this with a drywall sanding stick and the mesh sandpaper for sanding drywall mud. To limit dust, you can wet-sand, just remember to rinse and wipe frequently. Don’t let the slurry dry; it will glue itself back to the hull and be very difficult to remove.
Clean up thoroughly, recycling your plastic and washing off canvas tarps and drop clothes. Vacuum up the area to remove any dust and dispose of the debris according to your local regulations. Remember, even the dust is toxic. Remove any remaining tape.
Some areas of caution during bottom preparation and painting are the ground plate and other sacrificial anodes (zincs). Many boats equipped with a bonded ground system to deal with stray electrical currents and lightning strikes have a bronze ground plate on the hull exposed to the water well below the waterline. The metal terminal should not be painted as the coating will lessen the fitting’s conductivity to the water (earth ground). This is especially critical for a ground plate intended for lightning protection. In addition, sharp corners and edges are the best points of conduction for the plate, so try not to sand them round. Similarly, sacrificial zincs should never be painted.
Aaron Porter Taping the waterline assures a sharp line of transition between bottom and topside paints.
Next comes the fun part: painting the hull. I always paint the bootstripe first, so I have a smooth surface to tape to and get a crisp line between antifouling and topside finishes. This requires taping off again with a two-tape system. Wipe the surface down with an appropriate solvent per the TDS and apply the painter’s tape or other safe-release tape about 1/4″ (6mm) from the edge of the line. Then, apply the fine-line tape (I prefer 3M brand there) to the line and overlapping the safe release tape. Rub it all down with firm pressure. This technique assures the fine-line tape, which has superior adhesion qualities and resistance to some of the chemicals common in marine finishes, does not peel off any paint.
Once you’ve taped your bootstripe, you can immediately apply primer or paint. Do not leave the tape on any longer than about three days for successive coats. Hot coats (paint dry but not fully cured) can be applied per the TDS of your product. I tend to use 1″ (25mm) foam brushes for this, but a good bristle brush is fine. I have even sprayed bootstripes with a touch up HVLP spray gun. After applying the final coat, immediately remove the tape by pulling away from the still-wet paint at an acute angle. This prevents paint chips along the tape edge.
Wait until the paint has fully cured, usually a couple of days, before taping off for the antifouling, barrier, or prime coats. Tape the waterline again using the same technique in the previous step and suit up in full PPE. Apply the paint according to the TDS.
Aaron Porter Stir the paint thoroughly using a mixing attachment on a drill or a stationary paint mixer, and pour it promptly into a roller pan before solids start to settle out.
With most antifouling paints, you have to really move to maintain a wet edge; they dry quickly. The paint, which should be thoroughly mixed at the outset, will also require frequent stirring in the pan to keep the heavy solids from settling. Most commonly, you apply bottom paint by rolling it on perpendicular to the waterline, usually from waterline to keel unless you’re applying a hard shell (racing) type paint. It must be applied swiftly, keeping a wet edge between working areas. It helps to keep those working areas small and overlapping so the paint does not dry before you get to the next area. Also, apply more pressure on the roller toward the dry side of the application area. This helps reduce lines from the edge of the roller that overlaps with the area already painted. Also, flip the roller between strokes to keep the paint more evenly loaded on the roller.
Use a small brush to “cut in” around zincs, transducers, and other things not getting painted. It’s best to brush those edges first, then roll to the brushed coating. If your hull is lapstrake planked or molded, you will have to use a 3″ (76mm) roller running fore and aft following the planking and painting the edges with a small brush.
Aaron Porter First, cut in at the waterline and around hardware with a brush, then roll on paint to meld with the brushed areas (left). Roll the paint on in steady even passes from the keel to the waterline overlapping slightly with the last pass to assure coverage.
If you are using a hard racing finish, you may want one person rolling and one person tipping to assure a uniformly smooth brushed finish textured fore and aft. In addition to race-boat owners, some customers with classic runabouts or dry sailed boats that spend most of their time on trailers, demand a brushed finish on the hull bottom. If you need a second coat per the TDS, or the first looks a bit translucent, wait the recommended time according to the TDS before overcoating. (Note that water-based formulations tend to need an overcoat.) Then, immediately remove any tape in the same manner used for the bootstripe.
The next day, you will need to move the stands and blocking to prep/paint those areas. Keep a piece of cardboard, foam, or carpet between the blocks and your fresh paint. Let those patches dry as you do one more thorough cleanup and you’re ready to launch.
About the Author: Eric Renshaw grew up on the Great Lakes working on his father’s sportfishing charter boat. He started his boatbuilding and restoration business at age 19 and has built over 40 new vessels and restored more than 300 to date. Since 2020, he has been an instructor at Great Lakes Boat Building School (now the Marine Trades Institute) where he builds boatbuilders instead of boats. He continues to maintain boats in his home workshop and fish around the Straits of Mackinaw. He is a USCG certified boatbuilder and ABYC master technician.
