How to Paint the Bottom of a Boat

Co-authored by Travis Lund

Last Updated: May 29, 2022 References

A fresh coat of paint prevents aquatic life and barnacles from attaching to the bottom of your boat. Before you can start painting, however, you need to prep the bottom of the boat by cleaning, sanding, and priming the keel. Once that’s done, you just need to apply 2-4 coats of antifouling paint to the bottom. Painting the bottom of your boat once a year will keep it looking and working great for years to come.

Part 1

Part 1 of 3:

Priming the Boat

1
Remove the boat from the water. You must paint the boat in the air. You can get the boat out of the water by using an aeroplane wing. Back the trailer down to the water via a boat ramp. Have someone on the boat gently ease the boat onto the trailer using a light pressure on the throttle. Alternatively, you could hire a marina or boatyard to haul up the boat for you.^{[1]
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2
Clean the bottom of the boat. Use a hose to blast off any dirt. If there is algae or hard barnacles on the bottom, take a scour brush and scrub it off. You do not need to use any soap when washing the bottom of the boat.^{[2]
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- You only need to clean the bottom of the boat beneath the waterline. This is the keel of the boat. Look for the boot stripe, which is an unpainted border between the bottom paint and the topsides of the boat (which are the sides of the boat above the waterline).
- To remove stubborn or hardened grime, use a power washer. You can rent one at a hardware store. A boatyard may also have one on hand that you can borrow or rent.
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3
Remove the wax sealant if the boat has never been painted before. New boats will have a wax covering on the bottom. To get rid of it, buy a dewaxing solvent from a marine supply store. Dip a clean rag into the solvent and rub away the wax. Rinse it off using a clean, damp rag. Go over the entire keel.^{[3]
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4
Strip old paint if the previous paint job is significantly damaged. If the old paint job is still smooth, you don't need to strip the paint. If there are bubbles, large peeling strips, or ragged chips in the paint, remove the old paint entirely. First, brush on a chemical paint stripper over the old paint job. Use a 2 inch (5 cm) hook scraper to scrape off the paint.^{[4]
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- If you are stripping the paint, place a tarp underneath the boat to pick up the debris.
- Only strip paint beneath the waterline of the boat. Do not scrape paint from the topsides of the boat.
- If you keep your boat at a boatyard, ask the management if anyone there can use a power blaster on the boat. This tool should only be handled by professionals, but it can strip the paint much faster than doing it by hand. Ask for soda blasting on a fiberglass boat or sand blasting on an aluminum or steel boat.
5
Sand the bottom of the boat to prepare it for primer. Scrub the exterior of the keel with 80-grit sandpaper. It should have a dull “frosty” appearance when you are done.^{[5]
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6
Apply primer to the keel with a roller brush. Stir the primer with a paint stick. Prime the edges with a paint brush before filling in the middle with the roller. Make sure that there is an even coating of primer over the keel.^{[6]
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- You can buy a good boat primer at a marine supply store or online.
7
Sand down the primer once it is dry. It may take the primer 1-2 hours to dry completely. Use a fine grit sandpaper to sand the primed surface before you start painting.^{[7]
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Part 2

Part 2 of 3:

Applying the Paint

1
Buy antifouling paint to prevent barnacles and other water growth. Antifouling paint contains a chemical called a biocide, which will kill barnacles, algae, or other growth before it can develop on your boat’s keel. There are 3 types of antifouling paint that you can buy: ablative, hard bottom, and hybrid.^{[8]
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- Ablative paint is good for slower boats that are in constant use, such as fishing boats or pontoon boats. Ablative paint wears away on its own, which prevents you from having to remove the paint later on.
- Hard bottom paint is ideal for fast boats or boats that may not be used as often, such as speed boats. These paints don’t wear away very easily, but it is more difficult to remove them when you need to apply a new coating.
- There are “hybrid” or “semi-hard” paints that combine the benefits of ablative paint and hard paint. These are good for powerboats or frequently used boats.
2
Stir the paint with a stick. You can use a piece of wood or a paint stirrer. Stir the paint for at least 5 minutes. If you feel hard chunks on the bottom of the can, press down with the stick to break them up and keep stirring until the paint is evenly mixed.^{[9]
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3
Apply the paint to the keel with a roller. Fill about half of a paint tray with the paint. Dip the roller into the paint and roll it against the edge of the tray to distribute it evenly. Start painting at one end of the keel and move slowly towards the other. Use a paint brush to fill in small or awkward areas.^{[10]
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- Do not paint above the waterline. The topsides of the boat require a different type of paint than the bottom.
- If you need to add more paint to the tray, make sure to stir it in the can first.
4
Sand down the boat. The first coat should be dry by the time you finish rolling the paint. Take a fine grit sandpaper and gently sand down the keel of the boat again before you add the next coat.^{[11]
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Add a second coat to the boat. Go back to the beginning and use the roller to give a second coat of paint. This second coat will double the lifespan of the paint job.^{[12]
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- Some brands of paint may recommend that you do 3-4 coats total. If you do this, just remember to sand the keel in between each one.
- If you want, the top coat can be a different color than the bottom coats. This will help you realize when the paint is wearing thin.^{[13]
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6
Allow the paint to dry for several hours. Drying times can vary based on the brand of paint you are using. Read the can of paint to see how long you need to wait before putting the boat back in the water. In general, it may take several hours or overnight.^{[14]
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Part 3

Part 3 of 3:

Maintaining the Paint

1
Clean the keel every 4 to 6 weeks. While antifouling paint can help prevent barnacles and algal growth, it may not stop all of it. Make sure you wipe away any grime or growth that springs up on your new paint job as soon as possible.^{[15]
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- If you used ablative paint, bring the boat up out of the water. Use a hose to spray away any grime and scrub off stains with a sponge. You do not need to use soap.
- If you used hard bottom or hybrid paint, you can dive down below the water. Use your hand or a rag to wipe away any grime or algae. You can also hire a diver to clean the boat for you.
2
Use the boat frequently. Antifouling paints are designed to work better while the boat is moving. The more you use your boat, the more effective the paint will be. If you’re not planning on using your boat very often, you may want to store it on land.^{[16]
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3
Reapply paint once a year using the same paint as before. Mixing types of paint can reduce the effectiveness of the biocide. If you used an ablative paint before, stick with an ablative paint. If you used a hard bottom paint, keep using hard bottom. If you want to switch to a different type, you must completely remove all of the paint on the keel.^{[17]
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- If you’re unsure what type of paint was used before on the keel, take a wet rag. Wipe the rag along the existing paint. If the paint wipes off, it is an ablative paint. If not, it is a hard or hybrid paint.

Expert Q&A

Question

How do you maintain the paint and coating on the bottom of a boat?

Travis Lund
Sailing Instructor & Executive Director, Treasure Island Sailing Center
Travis Lund is the General Manager at the Vallejo Marina, a large marina located between the San Francisco Bay and the Delta in California. Sailing since he was six-years-old, Travis has over 15 years working in sailing operations and instruction and has pioneered a coaching platform that combined traditional coaching with multi-camera video support. He studied English at Michigan State University, where he was on the sailing team.

Travis Lund

Sailing Instructor & Executive Director, Treasure Island Sailing Center

Expert Answer

The best way to maintain the paint or coating on the bottom of a boat is to dry sail it, or store it out of the water. If the boat does need to be left in the water for a long period of time, it should be treated with an antifouling paint, based of the type of water the boat will be in. The boat should also be cleaned by a professional diver regularly, although the exact frequency will depend on the water where you live.