The Complete Guide to Marine Antifouling
When it comes to operating any marine vessel, protection and maintenance are paramount to continued operation.
Whether a small GRP cruiser, a fast RIB, or a high-use commercial craft, a vessels hull can become exposed to marine organisms over time. This in turn can not only lead to corrosion and degradation if not treated correctly, but also a reduction in speed and fuel efficiency.
A properly selected and applied antifouling system prevents deterioration from marine fouling, protecting the hull and ensuring that a vessel can operate effectively. This makes regular antifoul application an essential maintenance task for your boat.
With a high diversity of potential boat types as well as materials used in production, knowing which type of antifoul coating will be most effective for which vessel can be a challenge to navigate. As such, this marine antifouling guide will provide the technical insight and practical advice required to make the appropriate decision on your choice of antifoul, based on vessel type, water conditions, and surface material.
The Role of Antifoul in Marine Protection
Antifouling paints are a specialised coating that protect your boat/vessel from the buildup of organisms such as algae. They achieve this by releasing chemical biocides that grow to a boats hull.
Benefits of Boat Antifouling
Improved Fuel Efficiency
The absence of antifoul can lead to marine build-up on a vessel. This creates a hull roughness that requires increased work from a boat engine to operate at a standard level, increasing fuel consumption.
Maintained Hull Performance
Effective antifoul can help maintain a smooth, clean hull. This improves speed, handling, acceleration and fuel economy, all of which are particularly critical for vessels that operate at high or low speeds
Antifouling Compatibility
Further considerations for any paint system should also extend toward compatibility and how effective the different layers can function together.
When it comes to marine antifoul, one of the most important variables is ensuring compatibility between primer and antifoul. Failure to do so can lead to:
- Peeling and flaking of an antifoul coat
- Lifting, wrinkling, and cracking
- Blistering
- Discolouration
While all SML & Jotun brand primers and antifoul coats that we sell are compatible with one another, epoxy based primer systems such as Jotamastic, Penguard or SML Epoxy will require a tie-coat prior to antifouling. For this, we would recommend applying Jotun Safeguard Universal, Vinyguard Silveergrey 88 or SML Underwater Primer.
For those boats with loose, flaking and/or leeched layers of existing antifouling we would advise sanding back to a firm surface prior to applying a coat of either SML Sheild Underwater Primer or Jotun Vinyguard before two coats of the chosen antifoul. A third coat should also be applied at the waterline and leading and trailing edges.
Types of Antifouling Paint and Their Applications
Antifouling coatings can be categorised by their unique construction and behaviour in water. While the specific formulations can differ between brand, marine antifoul coatings are generally either: self-polishing (self-eroding) and hard (non-eroding).
Both have merits, and the correct choice depends on vessel speed, substrate, water type, usage frequency, and maintenance schedule.
Self-Polishing Antifouling
Self-polishing antifouling functions through a mix of soluble bindings and fresh biocides.
These biocides are specifically formulated to protect the hull from damage against saltwater organisms. As the antifoul coating dissolves in the saltwater, a fresh layer of biocides is continuously released, ensuring continued protection for your vessel.
There are different factors that vary product to product that can impact the specific uses and effectiveness of self-polishing antifoul such as:
- Time and speed moving
- The movement of water
- Water conditions, temperature, pressure and salinity
- The number of coatings applied
- Preparation and drying time
Due to the dependence of saltwater salinity for an effective dissolve, self-polishing anti foul products are best recommended for boats that operate in saltwater environments at around 30 knots or less.
Top Self-Polishing Antifoul Products
Jotun Mare Nostrum SP is an all-season self-polishing antifoul from Jotun that is compatible with various substrates such as steel, glass reinforced plastic and aluminium (only when using the white colour). It’s self-polishing properties make it best suited for small leisure crafts and commercial vessels with a max cruising speed of 30 knots.
Our SML CU-PRO antifoul coatings utilise commercial grade biocides for enhanced protection from saltwater organisms, providing up to 3 years protection. This ensures optimal performance and fuel efficiency for smaller leisure boats up to commercial crafts with a cruising speed of 30 knots.Jotun’s premium self-polishing -
Jotun Seaforce 90 - is a single component antifoul that provides up to 60 months protection and is suited for commercial vessels only. The self-polishing biocides provide extensive protection for aluminium and steel substrates when used with suitable primers and tie coats such as Jotun Jotamastic 90 and Jotun Safegaurd Universal ES respectively.
Hard Antifouling
Hard antifouling instead requires a tough coating that itself is not soluble.
It remains intact on the hull and releases higher concentrations of soluble biocides from the coat as seawater flows into the pores of the hard coating. This process creates the protective antifoul barrier.
Because they don’t erode, hard antifouls are ideal for high-speed vessels where water turbulence would otherwise strip the coatings prematurely. They’re also suited to boats that are dry-stored or kept on drying moorings, as the coating integrity is maintained when exposed to air. However, over time the build-up from repeated coatings can affect performance, so periodic removal may be necessary.
There are different factors that vary product to product that can impact the specific uses and effectiveness of hard antifoul such as:
- Time out of the water (as the used biocide cannot be washed away)
- Marine fouling challenges
- Thickness of application (impacting coat lifespan)
- Biocide solubility
Top Hard Antifoul Products
Jotun Racing antifoul is a hard matrix antifoul coating that provides single season protection and a smooth polishable surface for regular cleaning maintenance. It’s most effective for high-speed vessels and racing yachts in addition to steel and GRP substrates. It requires effective surface preparation for optimal performance.
Jotun Aqualine is a specially developed hard antifoul spray that cures to a physically hard and durable film. It is best paired with application on high-wear components like propellers, drives, and trim tabs, where it offers a controlled biocide release while resisting the damage from high-speed water flow and cavitation.
Hard or Self-Polishing Antifoul?
With both hard and self-polishing antifoul coats undergoing a unique chemical process during the release of biocides, each have unique uses and benefits that can be utilised to provide the perfect antifoul protection for your vessel.
Self-Polishing Antifoul
|
Attribute of Self Polishing Antifoul |
Example Use Case |
|
Consistent biocide release through diffusion maintains greater effectiveness throughout the season. |
Boats that move frequently at moderate speeds benefit from the controlled, sustained protection. |
|
Paint wear significantly reduces the need for expensive and labour-intensive stripping. |
This reduces the longer-term maintenance efforts and costs. |
|
The continuous polishing smooths out minor imperfections, helping to maintain a low-drag surface over time. |
Effective for vessels that consistently operate in areas of aggressive marine growth. |
|
The gradual loss of film thickness means the remaining layer is less prone to cracking and flaking from stress. |
For boat owners who operate within fluctuating temperatures, self-polishing antifoul reduces the potential stress of a fixed hard coating. |
Hard Antifoul
|
Attribute of Hard Antifoul |
Example Use Case |
|
Resists damage from high-speed turbulence, moorings, boat lifts, slings, and minor grounding. |
Boats that operate frequently at faster speeds (30 knots+) will benefit from the added hard antifoul protection. |
|
Less risk of damage from handling and temporary dry storage. |
Improved protection for vessels that aren’t permanently in the water. |
|
Can be wet-sanded to an ultra-smooth, mirror-like finish, providing maximum hydrodynamic efficiency and minimal drag. |
Beneficial for achieving the absolute minimum drag. |
|
Cure at much greater speed compared to self-polishing antifoul. This allows for faster application and return to operation. |
For boat owners who operate within time sensitive contexts and require fast coating. |
How to Match Antifoul to Boat Type, Usage and Substrate
When it comes to marine antifouling, it’s important to determine the most effective coating that is optimal for your boat type, substrate/material of application and how frequently the vessel will be used and require maintenance.
Below is a breakdown of what to consider and how to choose the most beneficial antifoul system for your specific vessel requirements.
Hull Material
Steel:
While steel as a material is highly robust, it is also a substrate increasingly prone to corrosion without adequate protection.
For the most functional result, a steel substrate paint system should contain an epoxy primer such as SML SHIELD HIGH BUILD EPOXY PRIMER, or Jotun’s Jotamastic 90 or Penguard range to protect the steel and create a stable base. A tie-coat should then be applied before applying antifoul. Most antifoul types are compatible if the correct primer and tie-coat system have been utilised.
GRP (Fibreglass)
GRP is chemically inert but requires mechanical abrasion to ensure adhesion. Clean the surface thoroughly and abrade with appropriate grit to create a profile.
For new builds or bare GRP, an adhesion-promoting primer such as SML SHEILD HIGH BUILD EPOXY PRIMER or Jotun Penguard HB may be is necessary. Most antifoul types are suitable to be applied on a GRP substrate.
Aluminium
Copper-based antifoul is not compatible with aluminium, as they create a galvanic cell that leads to corrosion. Use a copper-free antifoul such as SML CU-PRO (in white) after isolating the hull with an epoxy barrier system.
This ensures long-term protection without chemical incompatibility.
Usage Pattern
Occasional Use
Boats stored in marinas with seasonal summer weather use will benefit from a light self-polishing antifoul such as Jotun Mare Nostrum. This antifoul provides effective protection for the lower use throughout the year and allows for easy reapplication each year.
Regular Sea Use
For boats frequently travelling or frequently docked in fouling-prone areas, SML CU-PRO or Jotun’s Seaforce range of self-polishing antifoul offers a consistent biocide release that enables continued efficient operation.
Extended Service Commercial Vessels
For commercially operated vessels that need to stay in service across the year, Jotun Seaforce 90 offers proven reliability and protection and is suitable for aluminium and steel substrates. It is however only compatible with specific primers and topcoats.
Antifoul Application
For specific step by step guidance on the application process, view our antifouling application page.
Conditions of Application
When it comes to applying antifouling, it is important to ensure the surface environment is optimal for effective adhesion and maximised protective qualities.
Remove Flaking Paint
If you’re applying over existing coats, you should first look to remove all flaking or powdery paint. Follow this by degreasing and rinsing with fresh water to leave so the surface is clean, dry and dust free before application.
Marine antifouling over weak material will lead to coating failure, regardless of product quality.
Apply in Calm Weather Conditions
We would recommend you do not paint if rain is likely, in strong wind, or when the surface temperature is above 30°C.
Wind accelerates solvent flash-off and makes paint sticky to apply, while hot hulls prevent the coating from forming correctly. Always check ambient and substrate temperatures before coating.
Paint System Compatibility
If you are applying an antifoul to an old coating in which you may not be familiar with, we would always advise using a tie-coat to ensure effective adhesion and prevent any damage to the paint system.
Consistent Mixing
Heavy biocides such as copper oxide can settle quickly, so ensure the paint is being stirred thoroughly. Lift from the bottom of the can and re-suspend fully each time you refill the tray.
In cold weather warm the tin slightly for better flow. If heat or wind thickens paint in the tray, add only small amounts of the specified thinner to maintain rollability.
Effective Application Tools
Ensure you are using the most effective antifoul application tool. We would advise using a 4” or 6” solvent resistant medium pile roller, such as the SML blue stripe roller.
Troubleshoot Application Issues
Below we have included some of the more common issues that boat owners can face when applying antifoul as well as how you can remedy such issues.
Peeling or flaking
This typically occurs when antifouling is applied over loose, powdery, or heavily leached existing coatings. If the substrate is not properly prepared or a tie-coat is skipped over an unknown system, adhesion is compromised and the new film lifts away.
The remedy is to sand back to a firm surface, apply an appropriate underwater primer or tie-coat, and then reapply the antifoul.
Uneven Colour/ Finish
An uneven finish is almost always caused by poor mixing, especially with copper-rich antifouls where the heavy biocide settles at the bottom of the tin. Hot or windy conditions can also cause rapid solvent flash-off, preventing the film from levelling properly.
To fix this issue, try thoroughly remixing and applying an additional uniform coat under better conditions.
Premature Fouling
The premature occurrence of fouling usually indicates that the antifoul was applied too thinly or that high-wear zones such as the waterline and bow did not receive sufficient reinforcement.
It can also occur when self-polishing coatings are used on boats that remain largely stationary, preventing proper activation.
Increasing film thickness or selecting a higher-performance formulation typically resolves this problem.
Aluminium-Specific Issues
Aluminium substrate issues occur when copper-based antifouls are mistakenly applied, causing galvanic reaction and subsequent corrosion. Problems can also occur if the epoxy barrier coat is incomplete or has been sanded through.
To fix this issue, the affected area should be repaired with fresh epoxy barrier coating and recoated exclusively with a copper-free antifoul designed for aluminium.
Invest in the Right Coating for Long-Term Protection
When it comes to applying an effective marine paint system for your boat it important to ensure that you not only consider marine antifouling, but you are choosing the right type of antifoul.
By understanding how different antifouls behave, how they interact with various hull materials, and how environmental conditions affect application, boat owners can significantly extend hull longevity, reduce fuel consumption, and maintain optimal performance throughout the season.
With more than 20 years of experience supplying the marine market, our goal at SML Paints is to remove the uncertainty in product selection, system compatibility, and application technique.
For our full list of antifoul products view here, or call or email our team today for comprehensive marine paint help and advice.