How to Bottom Paint Your Boat

By West Marine Staff, Last updated: 3/23/2022

West Marine CPP paint with a person taping off the water line in the background.

Whether you’re painting your boat for the first time or just need a refresher for the upcoming season, we have the products and advice you need to get it done right. From the calculations to get exactly how much paint you need for your boat to compatibility charts between different types of paint and brands to make sure your job will last, we’ll walk you through everything you need to get the job done and get your boat back on the water.

Get Your Boat Out In Time

Boat being hauled out of the water

If you store your boat in a harbor your first step is making an appointment with the boatyard to pull your boat out and put it on stands. Space is usually limited so make your appointment as early as possible to make sure you get a spot. Once your boat is on stands make sure that you leave repositioning the stands to the professionals. Trying to move your boat yourself is dangerous and could lead to serious damage to your boat and injury to yourself.

Person bottom painting a boat on trailer stands

Trailer lifts make it easy to bottom paint your boat without taking it off the trailer.

If you own a trailerable boat then trailer lifts are a great option. The lifts allow you to raise your boat up to 12" above the bunks or rollers and support boats up to 10,000 pounds allowing you to bottom paint with your boat still securely attached to the trailer.

Be Ready when the Weather is Right

Paint manufacturers will list minimum and maximum application temperatures either on the can or on a data sheet for the paint you're using. If you don’t have access to a heated shed, plan to do the job when conditions in your area are closest to these parameters and buy your supplies before your boat is out of the water so you have everything you need to take advantage of good weather when you have it. You shouldn’t rush paint jobs, especially when you are working in the open air. If the weather doesn’t cooperate and it is too cold, too hot, too humid, or too windy to start a job, we recommend you wait until conditions improve. The results will be worth the extra time. Not only will the coat of paint look better, it will deliver better antifouling performance and will last longer.

Choose the Right Bottom Paint

West Marine bottom paint with painting supplies

How often and where you use your boat as well as what bottom paint is already on your boat are the main deciding factors for what bottom paint will work best for you. The two most popular paint types are ablative paints and hard modified epoxy paints.

Copolymer ablative paints have minimum overcoating times, and no maximum out-of-water time making them a good choice for trailerable boats that need antifouling protection while in the water, but also spend time on the hard. Not having a deadline to get your boat back in the water before the paint oxidizes also means that if you winterize your boat on the hard you can paint at haul out and relaunch in spring without having to paint.

Hard modified epoxy paints have a maximum amount of time that they can stay out of the water before the paint oxidizes meaning that you will need to get your boat back in the water quickly after painting. Hard modified paints are preferred by many long-distance cruisers and owners that keep their boats in the water year-round.

Water Based Bottom Paints are a great option as well as they are easier on the environment and on the person who applies them. They are also a great choice for applicators that are repelled by the strong chemical smell of solvent-based formulations and with ablative and hard modified epoxy types available there are options for any boat.

Know What Paint Is On Your Boat Already

Some paint formulas don’t play well with each other so if you are painting over existing bottom paint make sure that the new bottom paint is compatible with the old. Most for-sale listings will say when the bottom was last painted and what paint was used so you should know what paint is on the boat. If not, the easiest way to tell is to run your fingers along the bottom paint once it's out of the water. If paint comes off on your fingers then it is likely an ablative, if no paint comes off on your fingers then it is likely a hard modified epoxy paint. There are three general rules when painting over existing paint

  • Make sure the old paint is firmly attached. Don’t put good paint over loose, flaky paint.
  • Don’t apply paint over old paint that contains a slippery Vinyl or PTFE agent
  • Don’t apply a hard modified epoxy paint over a soft ablative paint.

If you know what paint is on your boat already and you’ve decided on the new paint you’re going to use, refer to the paint manufacturer’s compatibility guide to make sure your job will go smoothly.

Pettit Compatibility Guide

Interlux Compatibility Guide

Sea Hawk Compatibility Guide

West Marine Bottom Paint Guide

Figure Out How Much Paint You’ll Need

The next step is to figure out how much paint you’ll need for your boat. The following chart is a good starting point and accounts for two coats of paint which is the minimum number of coats required by most paints.

Boat Type Bottom Paint Topside Topside Polyurathane Enamel Waterline Deck
10' Dinghy 1 qt. 1 qt. 1 qt.
14' Outboard 1-2 qt. 1 qt. 1 qt 1 pt.
18' Runabout 2 qt. 1 qt. 2 qt 1 pt.. 1 qt.
20' Sailboat 3 qt. 2 qt. 2 qt 1 pt.. 1 qt.
24' Runabout 3-4 qt. 2 qt. 2 qt 1 pt. 1 qt.
30' Sailboat 1.5 gal. 3 qt 1 gal. 1 pt. 2 qt 2 qt.
32' Cruiser 1.5-2 gal. 3 qt 1 gal. 1 pt. 2 qt 2 qt.
36' Auxiliary 2 gal. 5 qt 2 gal. 1 pt. 1 gal. 3 qt.
40' Cruiser 2.5 gal. 6 qt 2.5 gal. 2 pt. 1.5 gal. 1 gal.
45' Sailboat 3-3.5 gal 8 qt 3 gal. 3 pt. 2 gal. 1 gal.
50' Sportfisher 4+ gal. 8 qt 3 gal. 4 pt. 2.5 gal. 1 gal.

Another way to determine how much paint you’ll need is to estimate the approximate surface area of the hull below the waterline using the following formula:

Length (From bow to stern) x Beam (The width of the boat, measured at its widest point) x .75 = Approximate Underwater Surface Area in Sq. Ft.

Each paint has a “theoretical coverage” area listed on the can, but keep in mind that most paints require at least two coats for sufficient coverage and protection.

Head to West Marine

Now that you’ve chosen your bottom paint, it’s time to pick up the supplies you’ll need to complete the job. We’ve created a checklist to help you make sure you have everything you need to do the job right.

Bottom Paint Shopping List

Bottom Paint

Make sure to get enough to apply two coats based on the formula earlier in the article and remember that for ablative paints it's recommended to add extra coats in high turbulence areas such as the bow, rudder and leading edge of the keel.

Masks, Coveralls and Nitrile Gloves

Bottom paint is toxic, make sure you have appropriate PPE to protect your skin and lungs.

Aggressive Scrub Pad

To help prepare the hull for painting we recommend the 3M Doodlebug™ Hi-Productivity Stripping Pad.

Roller, Roller Covers and Roller Pans

Antifouling paint is best applied with a roller. Spraying should only be done by professionals due to paint toxicity.

Fiberglass Solvent

Painting a newly manufactured fiberglass boat requires that any residual mold release wax on the hull be removed with a solvent. Use either Interlux 202 Fiberglass Solvent Wash or Sea Hawk S-80 Wax 'n' Grease Killer to prep for paint.

Thinner

Check directions on the can of bottom paint you're using for proper thinner type and amount.

Random Orbital Sander

Either a pneumatic or electric model will work.

Sandpaper and Sanding Discs

Sand where necessary to remove old paint and lightly between coats with 80 grit discs. Wipe off sanding dust with mineral spirits or one of the fiberglass solvents listed above if bare fiberglass is showing.

Masking Tape

Use wide tape for holding up masking paper and fine line tape for taping off the boot stripe.

Rags

Look for the recycled cotton T-shirt variety.

Paint Mixing Buckets

Drill-Driven Paint Mixing Tool

Paint Scraper

Use to remove old, loose or flaky paint before sanding.

Anodes

Since your boat is already out of the water it's a great time to replace your anodes. They are available in sets for specific makes and models of outboards and outdrives and also available for saltwater, freshwater or brackish water depending on where you typically use your boat. Other anodes include shaft anodes, rudder plate anodes, engine anodes, etc.

Prep the Bottom of Your Boat for Paint

Good preparation and priming are the basis for any paint job and antifouling paints are no different. Solid prep ensures good adhesion and better performance over time. If you are painting a new boat for the first time, wipe down the entire bottom with a good solvent wash such as Interlux 202 or Pettit Dewaxer to get rid of all mold release agents from the factory. To avoid just spreading the mold release agent over the hull, turn your cloth frequently and replace it often with a clean cloth. You can then lightly sand the hull with 120-grit sandpaper or use a no sand primer instead, such as Interlux Fiberglass No-Sand Primer or Pettit Sandless Primer. You may also want to consider adding a barrier coat to protect the hull from osmosis and fiberglass blistering.

Clean Your Hull

Person pressure washing the bottom of a boat

Power-washing works great to remove any remaining dirt or light fouling. For heavier fouling, use a strong acid-based bottom cleaner such as MaryKate’s On-Off. Be sure to wear eye protection, a good respirator and rubber gloves, as the cleaner is very caustic.

Deal With any Old Bottom Paint on Your Boat

Since most of us will be recoating over a previously painted bottom, we will need to make sure the surface is prepped for another coat. If old paint must be removed because it’s incompatible or too deteriorated to overcoat, be sure to have Aqua-Strip™, Ready-Strip® or other fiberglass-safe stripper system or other material on hand.

Sand the Hull

Person sanding the hull of a boat.

Once the surface has dried completely, we are ready to sand. Sanding is necessary to give the old surface some “tooth” to allow the fresh paint to mechanically adhere. Bottom paints are toxic by nature and most are solvent based, so you should always try to cover exposed skin, shield your eyes and wear a respirator to avoid breathing toxic fumes. We recommend outfitting yourself with a disposable coverall suit, gloves, high quality goggles and a dual-cartridge respirator as well as laying out a large tarp or drop cloth to cover the entire work area. Whether or not you know what paint is on your boat and what condition the paint is in are big parts of how much prep work is needed. Use the three cases below to determine what course is right for your boat.

If the old paint is known and in good shape: Remove old loose paint, dirt, grease, and marine growth with a power washer, brush or scraper. Wipe down with solvent wash. Sand with 80-grit paper. Exercise caution to avoid sanding through a barrier coat that may have been applied to prevent fiberglass blistering or damaging the gelcoat of the hull. Repeat solvent wash. Clean with the thinner recommended by your paint manufacturer. If you have to apply gelcoat blister protection, follow your manufacturer’s guidelines for surface and tie coat priming before you proceed with the paint application. If blister protection is not needed, you can apply paint directly to the sanded surface or the fiberglass.

If the old paint is unknown and in good shape: Clean, remove loose paint, sand (80-grit paper) and wipe down with a fiberglass solvent wash. Apply the recommended number of coats of tie coat primer such as Interlux Primocon or Pettit 6627 to ensure optimum paint adhesion. Then simply apply the antifouling of your choice following the manufacturer’s instructions. Some slippery Teflon paints such as the Interlux VC Offshore series may need to be removed before applying an incompatible paint.

If the old paint is unknown and in bad shape: Remove the old coats of antifouling paint. Use a paint remover that is compatible with the material of your hull. You may have to apply the paint remover several times to get rid of all the layers. If you are a racer or a stickler for a super-smooth bottom, the dreaded sanding longboard may have to come out. Once the paint is stripped, check for damage to the barrier coat that provides blister protection (if there is one) and patch it where necessary. If the hull does not have an epoxy barrier coat this is a good time to consider applying this protection. Then proceed with painting.

Tape Off the Water Line

Person taping off the water line

Use a high-quality masking tape to tape off the water line. This step ensures you'll have a nice clean edge when you're done painting. Don't skimp on your tape! Cheap tapes will often leave residue behind when you peel it off and may allow paint to bleed under the edge of the tape.

Roll On the Paint

Close up of bottom paint being rolled on

You’re now ready to start painting. Select the right paint accessories to match the type of paint you are applying. Rolling works best for most bottom paint applications, and a 3/8" nap solvent-resistant roller cover is the best match for most bottom paints. Don’t be tempted to try and use household-variety roller covers, brushes, or tray liners. Solvents used in bottom paints are much “hotter” than latex or oil-based household paints and will likely dissolve these applicators.

Thin-film paints are too runny for a heavy nap roller and are applied best either by rolling using a solvent-resistant foam roller cover or by spray with an airless sprayer. Make sure to get a few sizes of chip brushes to cut out around the masked areas and at the waterline.

Paint manufacturers’ instructions give single-coat square foot coverage and recommend the number of coats needed for optimum protection. Don’t try to economize on paint either by thinning it excessively or by rolling it on too thin.

Ablative paints in particular must be thickly applied. Apply extra coats in areas of turbulence such as the bow, rudder and leading edge of the keel. With copolymer and ablative paints, if you use a different color for the base coat, you’ll know it’s time to recoat when it begins to show through. Use the paint manufacturer’s coverage chart for an educated guess as to how much paint you’ll need to do the job. Get some special transducer antifouling paint to touch up any underwater transducers.

Use a drill-driven paint mixing tool to stir the paint right before you apply. If the paint has been sitting the copper may have settled at the bottom of the can so make sure to stir the paint throughly to get the copper evenly diustrubted throughout the solution. After thoroughly stirring the paint, pour into the paint tray and roll the paint evenly from one end of the boat to the other.

While Your Boat is on the Hard

Propseed on boat propellors and rudders

Painting and protecting other underwater assets is a great add on project while your boat is already out of the water. Propspeed is great for protecting underwater metals like outdrives from marine growth and FoulFree™ is great for protecting transducers. If your boat has underwater lights consider adding a coat of Lightspeed to them as well to keep them free of marine growth and shining bright all season.

Keep Drying Times in Mind

Freshly bottom painted boat on stands.

As you choose your paint and schedule your haulout, consider how much drying time you should allow between coats and how long the new paint can be left out of the water. Drying time between coats can vary from ten minutes for Teflon-based VC-17m to a 16-hour (or overnight) minimum for Trilux 33. For proper planning it is important to check the manufacturer’s recommendation in advance, both for the drying time and for the recommended number of coats. For example, West Marine’s most popular modified epoxy paint, BottomShield, has a drying time of four to six hours, and a maximum of 60 days before relaunch. Check your bottom paint for how long your boat can remain out of the water after the paint has dried and make sure you have it back in the water before then. Copolymer ablative paints have minimum overcoating times, and no maximum out-of-water time making them a good choice for trailerable boats that need antifouling protection while in the water, but also spend time on the hard. Recoat following manufacturer recommendations regarding overcoating times, which can vary based on temperature.

Once the paint is dry, remove all masking tape and clean up the area. Be sure to properly dispose of the old paint and any solvents used for clean up. If your boat is on jack stands or a cradle, painting under pads can be tricky. Depending on the drying time for the paint, you may be able to paint under the pads or bunks when the boat is on the Travel-lift for launch. Check with your yard. Some paints will dry adequately in the short time it’s “in the air”.

Time for Launch!

Once the paint has dried it's time to launch your boat, start your season and enjoy the water knowing that your hull is protected.