The Complete Guide to Bottom Painting Your Boat
A proper care and maintenance plan can help you enjoy your boat for many years. While there are many aspects of boat maintenance, we'll discuss one important element — bottom painting.
Bottom painting your boat is a preventive maintenance task that helps keep marine growth from attaching to the hull. Most boats don't come from the factory with bottom paint, so this is something you'll want to add to your boat maintenance list. If you use your boat regularly or keep it in the water for extended periods, a few coats of bottom or antifouling paint are crucial for optimal performance.
A clean, protected hull offers speed, safety and efficiency, while a fouled bottom can slow you down, hinder maneuverability and have you paying more at the fuel pump. Learn how to paint a boat bottom with our complete guide.
Conditions for Bottom Painting
Bottom paint your boat annually or as needed, especially if you use it often or regularly store it in water. You may be able to skip a year or so if you boat less frequently or store your vessel on land. Check the hull each year to determine if it needs a new coat of bottom paint.
When planning to bottom paint your boat, consider the weather and the paint's specific application guidelines. Keep in mind that bottom painting could take a week or longer from start to finish, depending on your boat's size, the type of paint you use and the environment you paint in.
Bottom painting on a day with favorable weather conditions will help you achieve the most effective paint job. Temperature, dew point and relative humidity are three important factors to consider.
Always check the paint manufacturer's specifications before bottom painting. Choose a warm, dry day without excessive humidity to keep your paint application smoother and to promote faster drying time.
Avoid painting on days with a high dew point or high relative humidity. The dew point is a measure of the amount of water vapor in the air. Relative humidity refers to how much water vapor is in the air at a given time compared to the maximum amount of water vapor the air can hold at a given temperature. When relative humidity drops, the dew point drops, and vice versa. Wait to bottom paint once all dew has evaporated.
With these considerations in mind, we recommend starting painting in the morning after the window for dew formation has passed. Avoid painting in direct sunlight, as the heat can cause the paint to dry too quickly.
Paint-Specific Application Windows
The type of antifouling paint you use also influences the timing of your bottom painting project. Depending on the paint manufacturer's specifications, your boat will need to go into the water within a certain timeframe after painting to activate the antifouling properties.
Some water-activated paints have generous launch timeframes, while others require relatively timely launching. Check with your paint manufacturer for specific launch times.
Choosing a Bottom Paint
Antifouling paints use a biocide to prevent marine life, such as mussels, barnacles, algae and weeds, from adhering to the bottom of your boat. There are two basic types of antifouling paint — soft antifoul and hard antifoul. Each of these paints releases biocide differently. Your boat's speed capabilities will help you determine the right paint to use.
When selecting an antifouling paint, think about how you use your boat and the type of paint, if any, that's been previously applied to the boat bottom to avoid compatibility concerns. While you can generally apply softer paints on top of harder paints, the reverse is not true. You should also only apply vinyl paints over existing vinyl paint, though some exceptions exist.
Always refer to a compatibility chart or speak with a paint professional to ensure you use the right products for your surface and application.
Soft Antifouling Paint
Soft antifouling paint is also known as self-polishing or self-eroding antifoul. It works by slowly eroding as the boat moves through water so a fresh layer of biocide is always present on the bottom of the boat.
Soft antifoul is suitable for boats that don't reach high speeds, as these can cause soft antifoul to erode too quickly. Check with the paint manufacturer for boat speed limitations. Operate boats with soft antifouling paint regularly to keep the biocide fresh.
Hard Antifouling Paint
Hard antifouling paint, also known as scrubbable or burnishable antifoul, works by continuously releasing biocides. Unlike soft antifoul, it doesn't wear off.
Hard antifoul is suitable for boats that reach higher speeds, as well as those with moorings that dry out. It's ideal for performance racing boats because you can scrub or burnish it to create a smooth surface with little drag.
Bottom painting your boat is a sizable project with several steps. When choosing a paint, refer to the paint manufacturer's instructions to determine the square footage a single coat will cover so you know how much to buy. The price of bottom paint varies depending on where you live and the availability of the paint. Different states have different guidelines and requirements related to bottom paint, which can influence which paints are available in your area.
You'll also need the following painting supplies as you work through the process:
- Paint thinner and solvents
- Paint tray and liner
- A solvent-resistant roller and roller frame
- Painter's tape
- 2- and 4-inch chip paint brushes for cut-in
- A drop cloth for messes
- Rags and tack cloths
- Paint buckets and stirring sticks
- A paint mixing tool
- An orbital or disc sander, electric or air-driven
- 80-grit sanding discs
- Fairing compound
- A plastic spreader
- A plastic fairing batten
- A painter's suit with a hood
- Foot covers
- Eye protection
- A vapor dust mask and/or a respirator for solvent-based paints
Make sure your painting accessories match the type of antifoul you're applying. You'll need rollers with short-nap, solvent-resistant roller covers. Don’t use household-grade brushes, roller covers or tray liners — the solvents in bottom paints are much stronger than the oil or latex paints used in the home and will likely dissolve these applicators.
Prepping the Boat for Paint
As with any painting project, careful preparation is crucial if you want to achieve the best results. You should clean, dry and prime your boat thoroughly before applying bottom paint. This might also include removing old bottom paint.
Consider your boat's material as you prepare to paint:
- Bare fiberglass: Bare fiberglass has a layer of mold release wax that will interfere with paint adhesion if you don't strip it. Wipe the hull with a dewaxing solvent, changing rags often. Sand the boat with a sandless primer or 80-grit sandpaper. Rinse off the sanding debris with fresh water.
- Bare wood: Working with a dry, clean surface, use seam compound to fill and smooth all seams. Sand the boat bottom with 80-grit sandpaper, and wipe it clean with water and a lint-free rag.
- Aluminum: Remove any rust or debris. Sandblast the surface until it's shiny and bare with a low profile. Use a vacuum to remove any sandblasting residue. Prime with a protective barrier, then wipe the cured surface with a de-waxer.
Recoating Old Paint
If you're applying new antifouling to existing paint, you'll need to prepare it to accept the fresh paint.
The first step is to clean the hull. Power washing is great for removing dirt, loose paint, grease and light fouling. Use a potent, acid-based bottom cleaner to remove more extensive marine growth, making sure to wear gloves, eye protection and a respirator.
The following scenarios outline how to prep your boat depending on the condition of the existing paint. We mention sanding here, but we'll cover it in greater detail in an upcoming section:
- If you know the type of existing paint and it's in decent shape: After cleaning the hull, wipe it with a solvent wash, then sand it using 80-grit sandpaper. Wash with solvent once more, then clean the surface with a thinner suggested by the paint manufacturer. Apply a protective gel coat if you want to avoid fiberglass blistering.
- If you don't know the type of existing paint, but it's in decent shape: Clean the boat to remove dirt, grime and loose paint flakes. Sand with 80-grit sandpaper and rinse the hull with fresh water. Apply a coat-tying primer according to the manufacturer's instructions to ensure the new paint adheres well.
- If you don't know the type of existing paint and it's in poor shape: Use a paint remover compatible with your boat's hull to remove the bottom paint. It may take several rounds of paint remover to cut through each layer. Once you've stripped all the paint, check the blister protection coat for damage and fill in any areas that need it. If your boat doesn't have a protective barrier coat, now is the time to apply one.
Sanding and Fairing the Hull
Once your boat is clean and dry, it's time to sand. The purpose of sanding is to rough up the hull so that it can grip the new paint. With a smooth surface, paint has nothing to adhere to and will wear off.
Sanding old bottom paint will expose you to toxic chemicals and fumes, so you must cover your skin and use a respirator and eye protection to stay safe.
Start by covering your work area with a dropcloth or tarp. Grab a dual-action, dustless sander or sanding block and gently sand the hull using 80-grit paper. Give extra attention to uneven or high spots, working to produce as smooth a bottom as possible.
Once you've sanded the hull, the next step is to fair it. Fairing a boat bottom evens out high and low areas in the hull to produce a level surface that's “fair” to the eye. The process can involve filling holes, reshaping voids or grinding down bumps so the surface is smooth to the eye and touch.
You'll use a fairing compound or fairing putty to fill and shape the boat. Fairing compounds typically contain epoxy resin and low-density fillers. Learn how to epoxy your boat hull with the steps below:
- Use a plastic spreader to apply the fairing compound to the boat bottom. Slightly overfill low areas so you can sand them down to an even surface later. If your hull is very uneven, use a notched spreader to apply the compound.
- Work with the fairing product to create your desired shape. This will help you avoid extra sanding after the compound cures.
- Blend and smooth it into the surrounding areas.
- To level the fairing product, adjust a plastic fairing batten so it fits the contour of the hull, and slowly drag it across the filled area.
- Let the fairing compound cure according to the manufacturer's instructions before sanding.
- You may need to fill and sand a few times until the low spots even up.
Taping Off the Borders
Start at a natural gap in the bootstripe line and apply long-mask tape as close to the bootstripe as you can, creating an even, smooth border. Remember to tape off through-hulls, marine struts, transducers and other metal elements because any copper in or touching bottom paint will create a reaction with metal and cause corrosion.
How to Paint a Boat Hull
Once you've taped off the borders, it's time to start painting. Have the paint shaken and stir it well just before applying.
Pour the paint into the basin of your paint tray. Dip the roller and unload the excess on the paint tray slope. Paint the hull, rolling up and down from the waterline to the keel. Work fast, as bottom paints can dry quickly. Stir the paint in the can before you refill the paint tray.
Once you've covered the entire hull, the paint may be dry enough that it's ready for a second coat. Check the paint manufacturer's specifications to determine how many coats to apply, as well as drying times between coats, because these can vary from ten minutes to a dozen or more hours.
Multiple coats extend the life of most bottom paints. You may want to apply one or two extra coats over bare hulls, around the waterline area, and any other areas that see a lot of wear and tear, such as leading edges of keels, skegs and rudders.
Letting the Paint Dry and Cure
Different bottom paints require different amounts of time to dry, so always refer to your bottom paint's instructions to determine how long to let your boat dry before moving it.
Additionally, some antifouling paints have activation windows, meaning you'll need to get your boat in the water within a certain timeframe to activate the antifouling agents in the paint.
Bottom Paint Your Boat With Fawcett Boat Supplies
Fawcett Boat Supplies has everything you need for a successful bottom painting project. Our extensive inventory of quality boat maintenance products, as well as our knowledgeable and helpful staff, will make your shopping experience easy and enjoyable.
Our goal is to keep you out on the water, whether you're an experienced boater or new to the hobby. Contact Fawcett Boat Supplies today to learn how we can assist you with your boating needs!