Priya Kapoor
Painting the bottom of your catamaran is a preventive maintenance task that helps to keep marine growth from attaching to the hull. It is critical to keep the hull free of marine growth to keep it performing at its best and to reduce fuel costs. Before applying bottom paint, it is important to clean, dry, and prime the boat thoroughly. This might also include removing old bottom paint. The choice of paint depends on the type of hull, water temperature, silt, substrate, pollution, water flow, and biofouling organisms present. For example, hard antifouling paint is suitable for boats that reach higher speeds, while soft antifouling paint is suitable for boats that don't reach high speeds. It is also important to wear proper personal protective equipment (PPE) when working with bottom paints due to the hazardous chemicals they contain.
| Characteristics | Values |
|---|---|
| Purpose | Prevent hard biofouling organisms like mussels, barnacles, and tubeworms, and soft growth like weeds and algae from attaching to the hull |
| Type of paint | Antifouling bottom paint |
| Paint selection factors | Water temperature, silt, substrate, pollution, water flow, biofouling organisms present, and the frequency and type of boating |
| Paint application factors | Temperature (50°–85°F), humidity (below 65%), and weather conditions |
| Surface preparation | Surfaces must be thoroughly clean, dry, and prepared; previously painted surfaces should be sanded with 80-grit sandpaper and cleaned with a solvent |
| Paint application methods | Brush, roller, airless spray, or conventional spray |
| Number of coats | 1-2 coats |
| Drying and curing | Dries hard and stays that way; does not wear away gradually |
| Personal protective equipment | Respirators, gloves, eye protection, face protection, skin protection, and appropriate clothing |
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What You'll Learn
Selecting the right bottom paint
Type of Bottom Paint
Bottom paints can be broadly categorized into two types: hard and soft. Hard paints, such as epoxy or non-sloughing varieties, remain relatively unchanged once applied to the hull. They are more suitable for high-speed boats that cruise at over 40 mph and are used regularly. Soft paints, also known as ablative paints, slowly wear away over time and use, exposing new biocides. Ablative paints are a good choice for boats that are not high-speed models and are used less frequently. Additionally, there are hybrid paints that offer a combination of hard and ablative properties, providing a semi-hard and semi-ablative finish.
Biocide Content
The main purpose of bottom paint is to prevent marine growth, and this is achieved through the use of biocides. Cuprous oxide is the most common biocide, and the effectiveness of bottom paint in preventing growth is generally proportional to its copper content. However, copper is not environmentally friendly, so it is important to consider more eco-friendly alternatives if possible. Some antifouling paints also include an algaecide to prevent soft growth like algae, slime, and weeds.
Water Conditions
The type of water and the specific growth found in the area where the catamaran will be used are important considerations. For example, in regions where a green "beard" of growth is common, an anti-slime agent such as zinc pyrithione should be included in the paint. Local knowledge is often crucial in determining the most suitable paint for the specific water conditions.
Temperature and Humidity
It is recommended to apply bottom paint within a certain temperature and humidity range for optimal results and performance. Manufacturers suggest an application temperature range of 50°–85°F (or even up to 90°F according to some sources) and humidity below 65%. Planning the painting work according to favourable weather conditions is essential for a successful outcome.
Surface Preparation
Proper surface preparation is critical to achieving excellent results. All surfaces must be thoroughly cleaned, dried, and prepared before applying bottom paint. Previously painted surfaces may require sanding, solvent cleaning, or the use of a paint stripper to ensure a suitable base for the new paint.
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Preparing the surface
Clean the Hull: Start by thoroughly cleaning the hull of your catamaran. If there is existing paint, make sure it is in good condition and free from any dirt, grime, or marine growth. You can use a pressure washer or a long-handled scraper to remove any hard growth or slime. Ensure the surface is completely clean and dry before proceeding to the next step.
Sand the Surface: Sanding is an important step in surface preparation. It helps create a rough texture on the surface, which enhances the adhesion of the new paint. Use 80-grit sandpaper or a similar tool to sand the surface evenly. If the existing paint is in poor condition, you may need to use a paint stripper to remove it completely. Wear proper personal protective equipment (PPE), including a respirator, during the sanding process to avoid inhaling dust or chemicals.
Solvent Cleaning: After sanding, use a solvent cleaner to remove any residue or contaminants from the surface. Dampen a lint-free rag with a suitable solvent, such as TotalBoat Special Brushing Thinner 100, and wipe down the surface. Ensure the surface is completely clean and free of any grease or oil before proceeding.
Bare Fiberglass Surface Preparation: If your catamaran has a bare fiberglass surface, it requires special preparation. Use a clean, lint-free rag dampened with a fiberglass-safe solvent, such as TotalBoat Dewaxer & Surface Prep, to wipe down the surface. Change the rags frequently to ensure effective contaminant removal.
Priming the Surface: Before applying the bottom paint, it is essential to prime the surface properly. Choose a suitable primer compatible with your chosen antifouling paint. Follow the manufacturer's instructions for application, ensuring an even and consistent coat. Allow the primer to dry completely according to the recommended time frame.
Inspect and Prepare the Boat: In addition to surface preparation, there are other important tasks to consider. Inspect and maintain various components, such as seacocks, rudder bearings, and the propeller shaft. Disassemble and lubricate seacocks, replace worn transducers, and inspect rudder bearings. These tasks will vary depending on your boat's specific needs and condition.
By following these steps, you will effectively prepare the surface of your catamaran for bottom painting. Remember to work in favourable weather conditions and take the necessary time to ensure a high-quality finish. A well-prepared surface will enhance the adhesion, durability, and overall appearance of the bottom paint.
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Painting tools and methods
Firstly, it is important to prepare the surface. If the bottom paint is cracking, peeling, or otherwise in poor condition, it should be removed. This can be done via a scraper-only approach, several days of sanding, or chemical strippers. If the previous paint is in good condition, it can be left as is, but it should be scuffed up. If you are sanding, it is important to wear a respirator and goggles to avoid inhaling dust. The recommended grit for sanding is 80-grit, moving down to 60 or 36 grit if stripping the paint down to the hull.
Once the surface is prepared, it should be thoroughly cleaned and dried before applying the bottom paint. Manufacturers recommend applying boat paint when temperatures are in the 50°–85°F range and humidity is below 65%. If using a solvent-based antifouling paint, use solvent-resistant roller covers, brushes, and paint trays. If the paint is water-based, use water-safe rollers, brushes, and trays. The paint can be applied via brush, roller, or spray.
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Safety precautions
Painting the bottom of a catamaran is a messy and tedious job, but it can be done safely by following these safety precautions:
Preparation
Before starting the painting process, it is important to prepare the work area and ensure you have the correct equipment. Choose a favourable time when the weather conditions are good—the temperature should be between 50°–85°F and the humidity should be below 65%. Manufacturers recommend painting in the morning after the window for dew formation has passed, on a warm, dry day without excessive humidity. Avoid painting on days with a high dew point or high relative humidity, as this will affect the drying time of the paint.
Equipment
Make sure you have the correct painting supplies. You will need rollers with short-nap, solvent-resistant roller covers. Do not use household-grade brushes, roller covers or tray liners, as the solvents in bottom paints are much stronger than household paints and will dissolve these applicators. You will also need safety equipment, such as respirators and protective clothing. Many bottom paints are toxic and contain pesticides, so it is important to familiarise yourself with the entire application process and follow all the required safety precautions.
Boat preparation
Before applying the paint, thoroughly clean, dry and prime the boat. This might include removing old paint and considering the boat's material. For example, if you are working with bare fibreglass, you will need to strip the layer of mould release wax, which will interfere with paint adhesion. Sand the boat with sandless primer or 80-grit sandpaper and rinse off the sanding debris with fresh water. If you are working with bare wood, use a seam compound to fill and smooth all seams, then sand and wipe clean with water and a lint-free rag. For aluminium, sandblast the surface until it is shiny and bare.
Time considerations
Factor in the possibility of "unexpected" extra time for chores, such as disassembling and lubricating seacocks, replacing worn transducers and inspecting rudder bearings. You shouldn't rush paint jobs, especially when working in open air.
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Eco-friendly alternatives
The chemistry of bottom paints is evolving, and with many new boaters and boat owners, boat surfaces need to be more effective and less harmful to the environment. The paint should be sticky enough to adhere to the hull but slippery enough to prevent marine growth from getting a grip. It should also be easy to apply and aesthetically pleasing.
There are several eco-friendly alternatives to bottom paints that are effective and affordable. Ultima Eco, for instance, contains 50% more ECONEA™ biocide than other options, providing multi-season performance. It has a slime-fighting agent and can be used on all substrates, including aluminum. Mission Bay Bottom Paint is another copper and solvent-free option that uses an enhanced, nano-based technology copolymer, which releases an active copper-free biocide when exposed to UV light. It can be used on aluminum hulls without a traditional barrier coat system.
Hydrocoat Eco is a water-based, copolymer ablative paint that combines a high level of metal-free ECONEA™ biocide with a powerful slime-fighting inhibitor. This paint is easy to apply and clean up, with low VOCs and no strong smell. It is compatible with all substrates, including steel and aluminum, and will not lose effectiveness when removed from the water. Pacifica Plus is another eco-friendly option that works well in high-growth areas and remains effective after the end-of-season haulout. In the spring, it can be reactivated with a light scrubbing or sanding.
Another innovative product is Adaptive Surface Technologies’ SLIPS®, which creates a super-slippery surface that prevents aquatic growth from sticking. It lasts 3-5 years and reduces fuel costs by 8% in powerboats.
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Frequently asked questions
Bottom paint, also known as antifouling paint, is a type of paint applied to the bottom of a boat to prevent the buildup of marine growth, such as barnacles, tubeworms, and algae, which can decrease the boat's speed and manoeuvrability and increase fuel costs.
It is recommended to paint the bottom of your boat annually or as needed, especially if you use it frequently or store it in the water for extended periods. You may be able to skip a year or two if you boat less frequently and store your catamaran on land.
The type of bottom paint you should use depends on various factors, including the type of hull, the water conditions, and your budget. Common types of bottom paint include soft/self-polishing antifouling paint, hard/scrubbable antifouling paint, ablative paint, and hybrid copolymer ablative paint. Consider factors such as boat speed, water temperature, and environmental impact when making your decision.
