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Boat bottom paint buyer’s guide



You’re perusing the chandlery looking for a gallon of boat bottom paint (a.k.a antifouling paint). You just want to get in, get out, and get on with painting your boat. But alas, you reach the paint aisle and you’re awash in choices and confounding terms: “hard modified epoxy, premium ablative, water-based, photoactive, thin film…” The shop assistant suggests a paint, but your marina neighbor swears by another brand. You’re feeling defeated. What happened to the good old days of slapping on some tar with a paint brush and calling it a day?

Welcome to the convoluted (and at times highly controversial) world of boat bottom paint!

Fortunately, antifouling paint is not all that complicated. It just seems that way because manufacturers have created a dizzying array of options and features. Here’s what you really need to know to make a decision:


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boat bottom paint


Antifouling paint is a specialized paint that is applied below-the-waterline to inhibit the growth of weeds, slime, and animals (e.g. barnacles and tube worms). All antifouling paints are made from a combination of these four components:

  • Biocide (deters living organisms)
  • Resin (keeps the paint together)
  • Solvent (determines application characteristics and cleanup/removal methods)
  • Pigment (color and viscosity).

The differences in paints are based on the sources and mixes of these four elements. For example, whether a paint is soft or hard is determined by the type, combination, and quantity of resin used in the mix. More on the differences between soft and hard paints later!


For boats kept in the water for more than a week at a time, it is necessary to apply boat bottom paint to prevent growth of marine life (fouling). This is because:

  • Fouling can dramatically reduce a boat’s performance. A fouled bottom causes drag, which increases fuel consumption, reduces speed and handling, and can even cause a boat to sit lower in the water.
  • Fouling can permanently damage the hull. In extreme cases, where growth has been left unchecked for a long period of time, the “glue” produced by fouling critters can damage fiberglass and wood.


Do your really need to paint or does your hull just need cleaning? Depending on the marine life in your area, how you use your boat, and how often you clean your hull, you may need to apply antifouling paint anywhere from once a year to once every three years. If you see any of the sings below, it’s time to repaint.

• If you can see the paint has worn through. A common practice is to alternate colors so you can clearly see when this has happened.
Flaking or blistering paint. When water gets under the paint (often around hardware) it can cause blistering and damage. If you see a fair bit of flaking or blistering, it’s time to repaint.
If you find yourself using abrasive tools and scrubbing to remove growth while cleaning your hull. Your dive service can also advise you on when it’s time for new paint.
If you notice performance issues, like reduced boat speed or handling and you have growth on the hull, fouling may be the culprit.
Corrosion (if you have an aluminum boat). Aluminum does not “rust”, though it does produce aluminum oxide which causes paint to flake off, exposing the aluminum underneath.
Your boat has been stored in the water for three seasons. Despite manufacturer claims to the contrary, most paint jobs don’t last more than three seasons.

boat diy


Most boaters apply antifouling paint at the beginning of the season once temperatures have warmed up enough for the paint to dry. It’s important to know that paint will only dry under the right conditions (generally speaking 50-85 degrees F and below 65% humidity). It can be too cold, hot, or humid for the paint to cure, so read the manufacturer’s directions and choose your timing carefully.

Note that “hard” paints lose their antifouling effectiveness when exposed to air so you would not want to apply a hard paint in the winter and then store it out of the water for several months.


Antifouling paints can by and large be classified into two groups: Soft paints (a.k.a. ablative and self-polishing) and hard paints (a.k.a non-sloughing or hard modified epoxy). There’s also an increasing number of soft-hard paint hybrids on the market. As mentioned previously, the difference between a soft and a hard paint is the type, combination, and quantity of resin used in the mix.


A soft paint is intended to gradually ablate, erode, or slough off, exposing fresh biocide when the boat moves through the water.


  • Reduces paint build-up (as it’s constantly wearing away) which means less sanding when you go to repaint.
  • Does not lose effectiveness when exposed to air.
  • If you can see the paint on the hull, you know it’s working!
  • Soft paints contain less copper but the ablating action means the copper is used more efficiently.


  • If the boat is docked for long periods of time (not moving) the paint will not effectively slough off and fouling will occur.
  • Ablative paints are a poor choice for fast boats because higher speeds quickly wear away the paint.

Generally recommended for:

  • Slow boats (e.g. cruising sailboats and non-planing powerboats) that are used regularly and boats stored out of the water for part of the season. While slow boats may choose either hard or soft paint types, many owners opt for soft paints due to the advantages of lack of paint build up.


Interlux Micron CSC Antifouling Paint, Black, Quart


A hard paint does not erode over time and instead the biocide leaches into the water. Water will gradually penetrate into the paint until the biocide has been completely used up.


  • Hard paints are great for fast boats (e.g. racing sailboats and powerboats) as their tough exterior does not erode with increased speeds.
  • Can be “burnished” (finely sanded and polished) to provide an ultra slippery and fast surface.
  • Often contain a higher copper content which means better antifouling performance.
  • May be scrubbed more aggressively than ablative paint when cleaning the bottom of your boat.


  • Hard paints build up over time with each additional coat unless vigorously sanded back each year. Eventually the paint may build to a point where it cracks and peels and the hull will need to be stripped back (a very time-consuming process).
  • Loses effectiveness when exposed to air.

    boat bottom paint

    Stripping back paint to gel coat is a multi week job and one of the downsides to hard paints which build up over time

Generally recommended for:

  • Fast boats (e.g. racing sailboats or planing powerboats) and boats that stay in the water year-round. Ablative paints won’t last long at high speeds and a hard burnished surface will be faster for racers.


Pettit Paint Trinidad SR Antifouling Bottom Paint with Irgarol, Blue, Gallon


Trailerable boats that are in-and-out of the water pose a challenge. A fast trailerable boat may be better off with no paint at all as hard paints don’t do well when exposed to air for long periods of time. If that’s not possible, a hybrid soft-hard paint like Pettit’s Vivid may be a better option.


Pettit Paint ViViD Antifouling Paint, Red, Gallon


Hull material

Boat hulls can be wood, fiberglass, aluminum, and steel. It’s important to ensure that a paint is appropriate for your boat’s hull material.
For example, aluminum boats must use specialized aluminum-safe paint. The cuprous oxide in regular paints will result in galvanic corrosion and damage an aluminum hull. Aluminum-safe paints use alternate biocides like zinc and cuprous thiocyanate. It also bears mentioning that, for the same reason, you should never paint underwater metal components (like a propeller) with cuprous oxide paints.

Examples of aluminum safe paints:
Interlux’s Pacifica
Pettit’s Ultima Eco

Interlux Pacifica Plus Bottom Paint, Green, Gallon

Paint compatibility

If you’re repainting a boat you must ensure that any paint you apply is compatible with the existing paint on the hull. West Marine, Pettit, and Interlux all publish compatibility charts for reference. As a rule of thumb, soft paints can be applied over hard paints but hard paints cannot be applied over ablative paints. To switch from a soft paint to a hard paint, the soft paint should first be entirely removed.

But I don’t know what the old paint on the hull is!

If you don’t know what type of paint is currently on the hull, you can at least determine whether it is hard or soft paint by rubbing the hull with a light-colored, damp rag. If the paint easily rubs off onto the rag, it’s likely ablative.

Local marine environment

Marine biologists estimate there are more than 4,000 known fouling species! Generally these species fall into one of three groups: animals, weeds, and slime.
Differences in water quality and temperature result in different types of fouling. Factors like nearby population centers, inflows from rivers and streams, the speed of currents, and the amount of sunlight affect the types and degrees of fouling in a region. In short, fouling can differ from one marina to the next.

The type of fouling in your region will inform the type of paint you choose. Given the number of varying factors, it’s a good idea to ask boaters in your marina what fouling they notice and what paint they use.

For example, fresh water areas like the Great Lakes suffer primarily from slime, and hard “thin-film” paint offer a super slippery, low friction finish to prevent slime from attaching to the hull.

Examples of thin-film paints:
Interlux VC-Offshore Hard-Vinyl Antifouling Bottom Paint, Red, Gallon

Eco-friendlier options and environmental regulations

The word “biocide” literally means “life-killer”. As such, the biocides used in antifouling paint are harmful to marine life (and not particularly healthy for humans to work with). It’s widely accepted that accumulated copper from boat bottom paint is detrimental to the marine environment. In states like California and Washington this has led to new environmental regulations regarding the sale and use of boat bottom paints. Many locales also limit copper accumulation by prohibiting in-the-water hull cleaning.

Paint manufacturers have begun to offer more environmentally friendly alternatives, such as water-based paints and paints containing photoactive materials and organic biocides. ePaint is a popular manufacturer of non-toxic, copper-free, and environmentally responsible alternatives to conventional antifouling paints. These paints are kinder to the environment and healthier for the person using them.

Examples of eco-friendlier paints:

Pettit Paint Hydrocoat Eco Ablative Antifouling Paint, White, Quart

We this information in hand you can now walk confidently into your local chandlery and make an informed decision on antifouling paint. It’s time to paint!