How to Paint a Boat Bottom

Boat Painting

While painting a boat bottom is not quite the same as painting a gel coat, there are some similarities.

Many manufacturers recommend that you work in temperatures and humidity closest to the dew point. Also, like with a gel coat, make a test board. It can be as simple as a 2′ x 2′ piece of plywood.

Hull Preparation

When you paint your boat’s bottom, the preparation is more important than the actual painting. If you skip the prep, you will probably not get a good long-lasting coat of bottom paint. If you follow the proper steps, however, a fresh coat of bottom paint will give years of protection and make your boat look new again.

The first thing you need to do is wipe down the hull with a solvent wash such as Interlux 202 or Pettit Dewaxer. This gets rid of any mold release agents that may have been used on the boat in the factory, and will help ensure that your paint adheres properly to the hull.

It is also a good idea to clean the decks of the boat, as well as any fittings that are not attached to the hull. Wipe down all areas of the hull to remove dirt, oil and marine growth. Next, you will need to sand the surface of the hull with 80-grit paper. Be careful not to sand through any barrier coat that is applied to prevent fiberglass blistering. If you have any bare fiberglass, it is a good idea to patch them before proceeding.

If you are using ablative or copolymer paint, it is a good idea to apply extra coats of the base color in areas of turbulence such as the bow, rudder and leading edge of the keel. These coats will keep the underlying paint from being damaged by the vibration caused by the propeller.

Before starting the painting process, be sure to pick a day with a low chance of rain. If you have to work outside, cover the area with tarps or plastic sheeting to protect it from windblown debris and raindrops. You will also want to be in a sheltered area, such as a shed or covered dock, where you can still take advantage of the sun’s rays.

If you are using two-part polyurethane bottom paint, it is a good idea to use a respirator with filter, goggles and a mask. This type of paint must be mixed and applied within a short time window, and can give off harmful fumes.


If you’re looking for a top-quality marine paint job that will last several years, you have to start with a quality primer. This layer, sometimes called an undercoat, forms the foundation of the entire paint system. It lays down the base for the subsequent coats of topside and bottom paint, as well as protecting the underlying substrate.

Primer paints come in a variety of formulations. Some are designed as a sealant, while others provide a filler that can hide small repairs and surface imperfections. The number of coats of undercoat required depends on a combination of factors, including the colour of the new paint. For example, a dark-coloured undercoat may require two or three additional topcoats than a light-coloured one.

For above-the-waterline applications, a 1-part acrylic or polyurethane primer provides a hard base that easily accepts a compatible topside finish coating. These types of primers are ideal for do-it-yourselfers and work well on fiberglass, wood, and aluminum surfaces.

Below-the-waterline, a different type of primer is required to prepare the metal or wood for an antifouling boat bottom paint. Designed to be resistant to hostile marine environments, these products are typically available in both water-based and solvent-based formulas. If using a water-based product, it’s important to choose a rust inhibitor to protect the metal from corrosion during the curing process.

A good primer will ensure the paint adheres to the hull and prevent water damage. It’s also essential to remove any hardware, like cleats and rails, or any teak or other wooden trim on the vessel, prior to priming. The boat must be washed from stem to stern, de-waxed, and thoroughly sanded before applying the primer and painting.

It’s also critical to follow the manufacturer’s instructions on sanding and drying times. Once the sanding is done and the primer is dry, the preparation stage is complete. A fresh coat of high-quality marine bottom paint will help your boat last longer than if it weren’t properly prepared. The result will be a beautiful and durable boat that looks like new for years to come.


Painting a boat is an expensive, time-consuming process. But if you do it correctly, your paint job will last for years. The key is preparation. As any good painter will tell you, 90 percent of the work is in preparing the surface. So, it is important that you carefully clean the entire hull before beginning any actual painting. Use soap and water and a scouring pad to remove any potential contaminants. This will help ensure that the new coat of paint adheres to the surface.

Once you’ve cleaned the hull, you can proceed to prep and sand the hull for your first coat of paint. Start with a good power washer and clean away all dirt, oil and marine growth. If you have the time, sand the hull with 120- to 180-grit sandpaper. But be careful not to sand through the gelcoat, which could leave you with a serious mess to clean up.

Next, you’ll need to determine what type of antifouling paint to use. There are many different types, so do some research to find the best one for your specific boating needs. For example, if your primary goal is to prevent fouling from barnacles, zebra mussels and other marine growth, you might want to consider a copolymer ablative paint. These types of paints release their biocide at a controlled rate, so you don’t have to worry about excessive sanding during haulouts.

You’ll also need to decide if you’re going to spray or brush the paint. Spraying is faster, but it requires a higher degree of skill to avoid runs and sags. If you choose to spray, be sure to wear a mask and fully cover any other nearby vessels with dust sheets to protect them from overspray.

Finally, be sure to carefully tape off any hardware you do not want to get paint on. If you skip this step, it will likely peel along with the paint once the final coat dries. And, if you’re not careful, you might have to go through the entire preparing and painting process all over again.


The topcoat is the final step in a thorough painting job, adding protection and beauty. Our one-part polyurethane paint offers superior self-leveling, stain and abrasion resistance, flexibility, UV protection, and color retention. It also has a gorgeous high-gloss sheen or can be converted to matte with an additive.

As with the primer and undercoat, before you begin applying a topcoat, the surface should be clean. Use a de-waxing solvent such as acetone to remove any wax or grease residue that remains on the surface. Then, wipe the hull with a soft cloth using a two-rag wipe on/wipe off method. Change rags often to avoid re-waxing the surface and reducing the quality of your finish.

It is best to plan your painting schedule ahead of time, allowing plenty of drying time between coats. Thin the paint according to the manufacturer’s instructions, based on substrate and air/hull temperature and humidity. It is important not to over-thin the paint, as this will sag when sprayed and can lessen gloss retention after curing.

Once the primer has dried overnight, sand the primer with 220-grit sandpaper on a hand sander with a foam pad. This will prepare the surface for painting, and help the primer adhere to the underlying hull. After sanding, wipe down the surface with the solvent recommended by the primer manufacturer (e.g. xylene or mineral spirits).

Then, sand the surface of the paint again with 320-grit sandpaper on a DA sander, to knock down any gloss and smooth out any runs in the product. Wipe down again with the solvent recommended by the product, and allow to dry.

Be sure to mask any areas that will not be painted, such as the area around the boat’s hardware. If this is not done, the hardware will eventually rust and start to peel paint off the hull, creating more work for you later on.

If you are removing an existing topside, it is best to strip the gelcoat first, using a chemical stripper such as our TotalBoat TotalStrip. This is a safe, easy-to-use, odor-free and biodegradable product that will not harm fiberglass or gelcoat.