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Top Bottom Paint FAQs

Here are answers to some questions customers frequently ask about bottom painting. If you have any other questions about bottom painting your boat, call our live tech support experts at 800-497-0010, or speak to a representative on the Jamestown Forum.

What's the best type of antifouling paint to use?

The short answer is the paint that works best in your home waters and for your particular type of boating. Things to consider when choosing antifouling paint include:

  • Water type: Is your boat in salt or fresh water? Is the fouling light, moderate, or heavy in your part of the harbor? Usually, the more copper an antifouling paint contains, the more effective the paint will be. Copper-free paints containing Econea have also proven very effective on hard fouling organisms like acorn barnacles. However, if slime and algae are a problem in your area, you'll want a paint boosted with an algaecide such as zinc pyrithione (this applies to copper-free biocides, too).

  • Water temperature: Typically, more biofouling occurs in waters that are warm and still, rather than colder waters where water flow is unrestricted.

  • Substrate: Certain antifoulings are better for wood, fiberglass, and primed steel; aluminum boats and aluminum outdrives and other underwater metals require antifoulings that don't contain cuprous oxide in order to prevent galvanic corrosion.

  • Environmental restrictions: Restrictions regulating VOC (volatile organic compound) and copper content in some areas of the country require the use of ablative antifouling paints that are water-based (instead of solvent-based) and/or copper-free.

  • Boat usage: How often do you use your boat, and what type of boating do you do? Brief boating seasons are OK for a single-season antifouling paint, but if you enjoy a longer boating season, it is probably more economical, and effective, to buy a multi-season paint. If you trailer your boat, consider a harder ablative antifouling paint.

  • Compatibility: If your boat has antifouling paint on it, you need to know whether the new antifouling paint is compatible with the previous antifouling paint. As a general rule, if you don't know what the previous paint is or you know that it's incompatible with the new antifouling paint, remove it completely before applying new paint.

    Formulas for antifouling paints are ever-changing. The chart below indicates generic compatibility guidelines and surface prep information for the common types of antifouling paint. These are general guidelines. Always consult the paint manufacturer's recommendations for specific compatibility information.

  • For specific compatibility when applying new TotalBoat antifouling paints over previous paints, see the chart below.

    TotalBoat Antifouling Compatibility Chart

    Bottom Paint Guidelines
  • Cost: How much do you want to spend? Typically, antifouling paints with more biocide are more expensive. Other properties that add to the cost of an antifouling paint include multi-season effectiveness, the ability to self-polish even when the boat is not moving through the water, and slime-fighting algaecide. At TotalBoat, we've created a full line of tested, effective antifouling paints for every DIYer's boat and budget.

For TotalBoat epoxy primers and antifouling paints, the TotalBoat Antifouling Paint Selection Chart shows which paint works best in certain conditions. For helpful answers to your questions about any of our products, please call and speak to an experienced specialist at 800-497-0010.

How much bottom paint do I need?

The best way to determine the amount of bottom paint necessary is to measure the surface area of your hull below the waterline. A reliable formula to use to calculate the surface area is:

Length x Beam x 0.85 = Approximate Surface Area sq ft

One important factor that determines how much paint you use is how heavy your application is. For the best antifouling protection, you will want to apply two coats, to the thickness recommended by the paint manufacturer. View the table below to see how much paint is needed for average boat sizes.

Type of Boat Bottom Paint
14' outboard 1-2 quarts
18' runabout 2 quarts
20' sailboat 3 quarts
24' runabout 3-4 quarts
30' sailboat 1.5 gallons
32' cruiser 1.5-2 gallons
36' auxiliary 2 gallons
40' sailboat 2.5 gallons
45' sailboat 3-3.5 gallons
50' sportfisher 4+ gallons

How many coats of antifouling paint do I need to apply?

With some ablative antifouling paints, 1-2 coats will suffice. The self-polishing copolymer ablative paints require 2-3 coats or more. Some people paint each coat a different color so they can tell where the paint has worn away most and needs extra coating next year. Or, if it's a multi-season paint, they can tell more easily when it's time to apply new antifouling paint. In many cases, the paint manufacturer will let you know if applying more coats extends the longevity and effectiveness of the antifouling paint. However, if ablative paints are applied too thickly, they'll crack, and hard antifouling paints applied to thickly will trap the biocide in the coating so it can't leach out.

How long do I need to wait before I can overcoat?

The overcoat times are specified in the documentation provided by the paint manufacturer. Pay attention to the recommended time before overcoating or the coating will not cure properly.

How do I know if my new antifouling paint is compatible with the old antifouling paint? See Compatibility section.

Do I need to remove the old antifouling paint before applying new paint? See Compatibility section.

Can I use antifouling paint on underwater metals like props and outdrives? To prevent galvanic corrosion, aluminum parts must be painted with an antifouling that does not contain cuprous oxide. Antifoulings that contain cuprous thiocyanate or zinc biocides are safe for aluminum. Copper-based antifouling paints can be applied to properly prepared props, outboards, and outdrives that are bronze or stainless steel. It's important to understand that the longevity of antifouling paint applied to underwater metals exposed to harsh conditions and treatment varies, especially for propellers. Also, be careful not to get antifouling paint on sacrificial zinc anodes, which are often located near prop shafts.

What kind of antifouling paint should I use on an aluminum hull? To prevent galvanic corrosion, use a copper-free antifouling paint.

Do I need a primer under antifouling paint? If your previously painted hull is in good condition with no chipping or cracking, and you're applying a compatible antifouling paint, typically you can just sand lightly before applying a new coat of antifouling.

If you have a new fiberglass hull, you will want to apply an epoxy primer and barrier coat first, before overcoating with antifouling.

If your fiberglass boat has gelcoat blisters, either fix the blisters yourself or have them repaired professionally. Then apply an epoxy primer and barrier coat product before overcoating with antifouling paint.

If you use an epoxy primer as a tie coat, or epoxy primer and barrier coat under your antifouling, be sure to apply the first coat of antifouling paint while the primer is thumbprint tacky. If the primer comes off on your thumb that's too soon. If you can't leave any thumbprint in the primer, you've missed the application window. If this happens, sand the surface with 80-grit sandpaper, wipe the surface clean, and apply antifouling, following the paint manufacturer's recommendations for thickness.

What is Econea? Econea™ (tralopyril) is a potent, copper-free biocide used in some antifouling paints to protect against hard marine fouling, such as barnacles and zebra mussels. It is very effective in low concentrations, and is safer for the environment than copper-based antifouling paints because when it breaks down quickly in the water, it's biodegradable. Because Econea is not copper, antifouling paints that contain Econea are safe for aluminum hulls and underwater metals such as propellers and outdrives.

Won't a water-based AF paint just dissolve and leave the hull unprotected while the boat is in the water?

No, once completely cured, water-based antifouling paint will not dissolve in water. As the water-based paint cures, the water evaporates and leaves behind a protective film of biocide, pigment, and (if included in the formula) algaecide.

Do I need to thin antifouling paint?

To be effective in preventing biofouling, antifouling paints are made to be applied right out of the can (after thorough mixing to incorporate the biocide that may have settled to the bottom). Thinners are not recommended. Yes, the paint can be expensive, but resist the temptation to add thinner in an attempt to increase the coverage area of the bottom paint because the coating will not be thick enough and will fail to perform as expected.

How long do I need to wait before splashing the boat?

For most antifouling paints, it's required that you wait at least overnight to launch the boat. At any rate, it's important to follow the paint manufacturer's recommendations carefully, otherwise the antifouling coating can become detached in the water if it hasn't had time to cure properly.

Can I use bottom paint on my topsides?

It's not recommended. The first reason not to use antifouling paint on topside surfaces is UV exposure, which causes instability and fading in antifouling paints. For example, have you ever noticed antifouling paint that is above the true waterline? The UV exposure makes the exposed bottom paint color looked bleached out, especially if it's red.

For cosmetic reasons, antifouling paints dry with a flat finish that is not pretty on topsides, which are usually glossy and/or brightly colored. And anywhere the paint gets wet, the copper in the AF paint will change the color, making it inconsistent with the rest of the finish, and, well, ugly.

Another reason not to use bottom paint on topsides is it would be messy— if you touch or rub up against ablative paint, it'll rub off on you or onto whatever touches it.

Finally, antifouling paint is expensive, and is formulated to deter biofouling organisms on the underside of the hull. In essence, you would be applying a coating that had more oomph than you need. At least until barnacles can fly. Bottom line, it would be expensive overkill to apply bottom paint on topsides.

Can I spray bottom paint?

Antifouling bottom paint is very thick, much thicker than topside paint, for example. Thinning is not generally recommended. Because of this, and because of the toxicity of the biocides and solvents in the paint, bottom paint is typically applied by roller or brush, although some bottom paints can be sprayed. Spraying bottom paint is typically done by professionals with proper personal protective gear, equipment, masking, tenting, and ventilation.

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