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Bottom Paint Application and Maintenance

Bottom Paints – What is going on down there?

Bottom paint is usually one of the last things boaters think about when it comes to maintenance; however, the upkeep of your boat’s antifouling paint is very important. In this article we are going to cover all aspects of applying and maintaining your boat’s underwater antifouling paint also known as bottom paint. Marine antifouling paints help protect your boat's hull from growth of unwanted marine organisms such as algae (slime), barnacles and zebra mussels. We will also go over step by step instructions to help you get the best results when painting the bottom of your boat. Maintaining your boat's painted surfaces and periodically repainting the boat is important for more than just your boat's appearance. Without the antifouling paint the bottom of your boat will become heavily coated with marine organisms in a short period of time. The accumulation of barnacles and other growth will cause performance loss due to the increased drag. The very first type of bottom protection in the days of wooden clipper ships consisted of copper sheets nailed to the bottom of the vessel. During the 20th century the preferred type of bottom paint was a chemically based product using metals like tin and copper and other compounds which sometimes contained elements that are not only effective at keeping marine organisms from growing on the boat, but they also have had tremendous negative impacts on the marine environment as a whole. In recent years those products containing TIN, Tributyltin and organotin have been banned world wide.

New Bottom Paint Technology Environmentally Friendly

Much advancement has been made in the last decade by the Flexdel Corporation the maker of AQUAGARD water-based antifouling paint. Starting in the year 2000 Flexdel began a research and development program aimed at developing an environmentally preferable coating for the marine industry. The program which was funded by Flexdel and a Commission of Science and Technology grant has achieved its goal of reducing copper from marine antifouling paint. The result is a product that effectively prevents growth of marine organisms while offering the safety of low levels of volatile organic compounds (VOC’s) and a significant reduction of heavy metals compared to other competitors. The new AQUAGARD product line is a co-polymer ablative paint with the ablative feature being a key part of how the system works. Ablative paints continually leach out the biocide which is the active element used to prevent barnacles and other organisms from growing on your boat. AQUAGARD still uses cuprous oxide (copper) as its biocide; however the manufacture has reduced the amount and perfected and controlled the rate that it leaches out. The result of the entire R&D project is a bottom paint that requires less copper yet still provides year-round protection. This new product is water based and it produces better flow and rolling ability when applied; and since there are no toxic solvents used in the paint, you can apply it indoors without any harm to your heath. I can tell you first hand that applying old style solvent based bottom paint is not good for your body even when applied outside in a well ventilated area: the smell of the solvents gets into your clothes and doesn’t go away.

Your Bottom Paint Weekend DIY Project

Bottom painting your boat is a great do it yourself project if your vessel can be hauled out and blocked up, and if the marina where you keep the boat allows for this type of work. Generally the best time to clean and prepare your vessel for a bottom job is at the end of the season. However the job can be anytime of year as long as adequate time is given for the materials to cure before re-launching the boat. Be sure to closely follow the manufacture’s recommendations regarding material application, handling, and safety. Most bottom painting projects can be completed over the course of one or two weekends depending on the boat size.


The first step in the process is to thoroughly pressure wash the entire area to be painted. Once you let the slime and growth dry and harden it is much more difficult to remove. Go over the entire surface using a 5 in 1 tool or scraper to remove any barnacles or other heavy growth. If the bottom paint is old and starting to flake off, you should scrape and remove all loose flakes. You might want to consider using a paint stripper and removing the old layer completely if the paint is in poor condition. During this inspection process be on the lookout for blisters or any other fiberglass damage that may require repair prior to commencing with the application of new bottom paint. Remember that the adhesion of the new paint is only as good as paint that it is being applied over. So watch for signs of adhesion failure. Anywhere the old paint is flaking or lifting, the bottom needs to be stripped or sanded. You may also have to strip the bottom if you are changing the type of paint. For example, the aggressive solvents in vinyl paints lift other types of bottom paints, so if you are applying vinyl, any non-vinyl paint has to come off. And soft, sloughing paints are a poor undercoat for anything other than a fresh coat of the same.


Antifouling dust can be a major health hazard and you really should be wearing a respirator and appropriate cartridges as well as eye and ear protection, gloves and a full coverall paint suit for your own safety.

When to Strip Old Paint:

Adhesion of the new paint is only as good as the paint that it is being applied over. So watch for signs of adhesion failure or loose and flaking paint. Anywhere the old paint is flaking or lifting, the bottom needs to be stripped using a liquid stripper. It may also be necessary to strip the bottom if you are changing the type of paint. Soft, sloughing paints are a poor undercoat for anything other than a fresh coat of the same.


If your bottom paint is in fairly good condition and is not the type of soft paint we mentioned above then you really only need to sand it thoroughly with 80-grit sandpaper (you can use a finish sander or an electric random orbital sander), clean it up, tape it off and you’re ready to start applying fresh paint. Be sure you leave all zinc anodes unpainted. If you are installing new zincs make sure you do not paint over their mounting locations. Good electrical contact is essential for zincs to do their job. Always wear all of your personal protection gear when sanding or handling antifouling paint!

Bare Fiberglass:

Any hull that has not been previously painted has a coating of mold-release wax on the fiberglass, which will interfere with paint adhesion unless you remove it. Clean the hull surface thoroughly with de-waxing solvent and plenty of clean rags. Prior to sanding be sure to mark and/or tape-off the water line. Sand the de-waxed hull lightly with 80-grit paper. Wipe down the area again to remove all sanding residue. Now it is time to do your very best tape and masking job using tape that is designed for this application. Some things to keep in mind when selecting a tape: (1) General purpose tapes are only designed to be left on a surface for 24 hours or less, unless you want a lengthy job of trying to remove the tape! (2) Select a "long-mask" tape for bottom painting, especially when you'll be applying multiple coats. (3) Now apply the appropriate primer as recommended by the paint manufacture that you have selected, and follow the manufacture’s recommendations for primer application. You are now ready to apply bottom paint.

Paint Mixing:

Antifouling bottom paint is often heavy and the copper will settle to the bottom of your can. Make sure you take the time and effort to mix your paint properly. If the copper isn’t evenly distributed, some areas of your hull will not be protected. If you have a paint shaker, run it for at least 5 minutes to get the copper and the pigment evenly distributed throughout the paint. If you are doing it by hand, keep dredging up the copper off the bottom of the can. Each time you add to your roller tray you will want to stir again to ensure consistency.


Roll the paint onto the hull using a short-nap roller cover (usually 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. Wear sleeves and gloves to keep the paint off your skin. Don't add any thinner to bottom paint unless the manufacturer specifies otherwise. Fill your paint tray with paint. Dip your roller and roll it up and down on the hull. Each time you refill the paint tray, first stir the paint in the can to keep the copper in suspension. By the time you work all the way around the hull, many bottom paints will be dry enough to overcoat. Check the specifications on the paint you are using. A second coat is usually recommended and will increase the life of almost any bottom paint; copolymers benefit from 3 or 4 coats. No sanding or other prep is needed between coats. Once the paint dries to the manufactures specifications, you are ready to launch.