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Do-It-Yourself: Bottom Painting


Bottom Painting

If you haul your boat out for the winter, applying several coats of antifouling paint is one of the rituals of Spring Commissioning. Most boaters find bottom painting messy and tedious, but it’s one of the key preventative maintenance jobs that keep your boat performing its best. A clean hull is safe, fast and efficient, and a fouled bottom cuts your top speed, damages maneuverability and lowers your fuel economy.

We’re going to look at what products you need to keep your boat’s hull protected, clean and free of critters. We’ll also discuss how to prep the hull, apply the paint, and the question of whether to apply an epoxy barrier coat. Knowing which products to select and how to use them can save you hundreds of dollars below the cost of having the work performed by a yard. For those in northern climates who have scheduled launch dates, getting boatyard work completed in time for your launch can also be a challenge. Doing the work yourself gives you satisfaction and peace of mind of a job done right.

Skill Level Description
Selecting the Correct Bottom Paint2 out of 10
Fixing a blistered bottom and applying a barrier coating7 out of 10
Prepping an existing bottom for painting3 out of 10
Painting your hull with bottom paint3 out of 10
Tools Required Shopping List
Selection of the Correct Bottom Paint

Fixing a blistered bottom and applying a barrier coating

Personal Protection

  • Paint suit with hood
  • Goggles
  • 3M 5000 or 6000 Respirator with appropriate cartridges
  • Gloves
  • Booties

Tools Required

  • Right angle grinder or sander, (dustless preferred)
  • Small diameter for blisters: 3M Green Corps Roloc Disks
  • Large diameter to remove bottom paint: 3M Gold Sanding Discs
  • Paint scrapers
  • Putty knives
  • 3M 80 grit sanding discs

Applying Bottom Paint

Tools Required

  • Paint tray, liner, roller frame, roller
  • All solvent resistant
  • Fineline and Blue masking tape
  • 2" and 4" chip brushes for cut-in
  • Drop cloth for spills
  • Rags and tack cloths
  • Paper buckets

Paint Application Items

  • Abrasive Discs
  • Paint Brushes & Rollers
  • Masking Tape & Paper
  • Abrasive Accessories & Scraper
  • Rags, Gloves & Buckets
  • Personal Safety
  • Abrasive Sheets
  • Abrasive Long & Skinny
  • Sandpaper

Paints & Solvents

  • Bottom Paint
  • Specialty & Nonskid Paint
  • Primers & Undercoats
  • Thinners & Solvents
  • Paint Removers
  • Barrier Coats

Overview

  • Plan ahead
  • Paint selection
  • Bottom prep
  • Applying the paint
  • A word about timing
  • Final thoughts
Bottom Painting

Plan ahead

Planning helps make paint jobs smooth and economical. You can’t change the weather, but you can pick a favorable time when the conditions are good. Manufacturers say it’s best to apply paint when temperatures are in the 50°– 85°F range and humidity is below 65%. 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.

Before haulout, perform other maintenance operations and inspections and check your boat’s electrical system because improper grounding can neutralize the effectiveness of antifouling paint. When hauled out for a bottom job, allow extra time for chores such as disassembling and lubricating seacocks, replacing worn transducers and inspecting rudder bearings. You may want to pull and inspect the propeller shaft and bearings.

Be patient

You shouldn’t rush paint jobs, especially when you are working in open air. Sometimes the weather does not cooperate. It may be too cold, too hot, too humid, or too windy to start a job. In that case 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 it will last longer.

Paint selection

Antifouling paint keeps marine organisms, taking the forms of shell (animal fouling from barnacles and zebra mussels), weed (plant growth) and slime (single-celled algae) from attaching themselves to your boat by releasing toxic biocides at a controlled rate. Copper compounds (cuprous oxide or cuprous thiocyanate) are most popular; and conventional thinking is that the higher the copper content, the more effective the antifouling paint.

Choosing an antifouling paint is regional, as boaters in the Great Lakes, Pacific Coast, Southeast, Gulf Coast and other regions tend to choose similarly to their neighbors in the local marina. Your choice also depends on how you use your boat and the type of paint you applied in the past (since there can be compatibility problems between some types and others). In general, softer paints can be applied over harder paints but not vice-versa. There are exceptions however, so it’s always best to consult a compatibility chart. Interlux and Pettit each make a full line of excellent bottom paints, including five primary types of antifouling paints. Let’s review each type.

Copolymer/ablative paints

Copolymer paints release biocide at a constant controlled rate throughout their lives, wearing away or “ablating” much like a bar of soap. Paint wears off faster in higher drag areas on the hull and appendages. These paints work well in high-growth areas and continue to be effective after haulout at the end of the season. In the spring, the paint is reactivated with a scrubbing or light sanding and you’re ready for another season. This is a huge time-saver for those living in northern climates. While the percentage of copper in the paint is important for evaluating its effectiveness, copper content is not the only consideration. Controlled polishing, the technology that controls how quickly the paint wears away, is another way we measure the effectiveness in a copolymer ablative paint.

Copolymer paints, such as West Marine PCA Gold, Interlux MicronCSC and Micron Extra, offer true multi-season protection, lasting as long as there is a reasonable coating thickness. Because they expose new biocide until the coating is worn completely away, additional coats add to their longevity. Pettit Hydrocoat offers the added benefit of being a water-based product with no solvents, an environmental plus, so there are fewer fumes to protect yourself from and easier clean-up using only water.

White copper for bright colors

Two copolymer paints, Pettit Vivid and Interlux Trilux 33, use a different biocide compound, cuprous thiocyanate or white copper that produces the brightest colors, whitest whites and blackest blacks. White copper also requires 50% less content than the heavy, dark copper used in conventional antifouling paints. Like most other paints, these types can be applied to fiberglass, wood or properly primed steel, and they are the only copper-based paints that are compatible with aluminum. Since copper and aluminum are dissimilar metals, applying a traditional copper paint directly on an aluminum hull would be disastrous. Galvanic corrosion would begin immediately and the hull would quickly deteriorate. For best results, aluminum boat owners should still prime their hull with an appropriate zinc-chromate or strontium chromate primer such as Pettit AlumaProtect before applying either Vivid or Trilux 33.

Modified epoxy

If you keep your boat in the water year round you are most likely a candidate for a high-copper-content modified epoxy paint that prevents growth by leaching biocides upon contact with water. Contact leaching paint releases the biocide at a steadily decreasing rate, leaving the hard coating of the original thickness at season’s end. Higher copper content, rather than the type of paint binder as with ablative paints, generally means greater effective performance in this paint type. Modified epoxy paints adhere tenaciously to most surfaces, and can be applied over most types of paints. Since these types of paints do not wear away, build-up will occur with each new coat and eventually your hull will need to have the coating stripped.

We used blue Pettit Trinidad SR on our 30' Newport sailboat, kept in a year-round marina and had great results with the product. It’s got 60% copper, and after two years we expect at least another year of service in our moderate-fouling environment, with light scrubbing by a diver every two months (along with a check on our underwater zinc anodes). Interlux Ultra is a similar high-copper paint.

Modified epoxy paints are the economy single-season choice if you’re in the Northeast or other regions where you dry-store your boat each winter. Two affordable and popular modified epoxies are West Marine BottomShield and Interlux Fiberglass Bottomkote NT, which are budget choices compared to more sophisticated ablative paints or high-copper modified epoxies. They offer hard, durable finishes with between 25% and 28% copper content.

Bottom Painting

Composite Copper Technology (CCT)

Replace the core of traditional cuprous oxide in a modified epoxy paint with environmentally friendly silica, reducing the copper footprint by up to 40%. The result is a copper biocide surface that provides superior protection for your boat, with a core that is made from materials found in the ocean. Our West Marine BottomShield features Composite Copper Technology, which is also environmentally beneficial, releasing 25% less copper metal into the environment.

Thin-film paints

Freshwater locations like the Great Lakes are plagued primarily by algae, and thin film paints such as VC 17m Extra and are a good choice for these environments. They offer a super-slick Teflon® finish to keep growth from attaching to the hull, in addition to one or more biocides. These paints dry almost instantly, so you can rapidly apply several coats and launch almost immediately. Because they are so thin, they must be applied with either a solvent-resistant foam roller head cover or by spray with an airless sprayer.

Sloughing and vinyl paints

There are two other types of paints we’ll mention. Neither is as popular as today’s modified epoxies and ablatives, but you should still know about them, especially if you happen to buy a boat with either type on the hull. Soft or “sloughing” paints used to be very popular among cruising sailors in northerly climates because they are very economical and last for a little longer than a season. Like Ablative paints, old-style sloughing antifoulings wear away as the boat moves, so the more often the boat is used and the faster it goes, the quicker the paint wears off. Sloughing paints are very soft, so few paints can be applied over them.

Vinyl-based paints are slick, hard paints that can be burnished to provide a smooth surface preferred by owners of performance boats. Unlike thin-film Teflon paints, vinyl paints like VC Offshore can be used in salt-water, although their antifouling effectiveness is inferior to the better quality modified epoxies and ablatives. Vinyl based paints are the least compatible, as they cannot be applied over any paints other than an existing vinyl paint. Very thorough sanding is required to achieve a good bond when painting over vinyl coatings with other types.

Paint for underwater metal

Aerosol antifouling paints, our final type, are sprayed onto underwater metal such as outdrives and props, which require an alternative to cuprous oxide as the antifouling agent to avoid galvanic corrosion from metal-to-metal incompatibility. Surfaces need to be primed before the paint is applied. Interlux Trilux 33 and Pettit Alumaspray Plus use different biocides, but both offer good protection from fouling. Alumaspray Plus is also only available in black, but Trilux 33 is offered in three different colors.

“Boosted” with slime-fighters

Today’s paints often contain a second “booster” biocide that prevents algae-related slime from growing at your boat's waterline. These “slimacides” act like sunblock for your hull, screening out the UV radiation that lets slime grow. Look for slimacides on your favorite paints by their trade names of Biolux or Irgarol.

The future: non-copper paint

Take a look at our West Advisor on Top Ten Antifouling Paint Buying Questions for a preview of what the future might look like. Paint manufacturers believe that ECONEA™, a metal-free antifouling agent developed by a pharmaceutical company, may be the future of antifouling paint. ECONEA-based paints like Interlux’s Pacifica Plus and Pettit’s Ultima ECO are now available at a competitive price. The products appear to offer many advantages over today’s copper-based paints.

Bottom Painting

Bottom prep

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 decided to tackle an epoxy barrier coat project, a light sanding with 120-grit sandpaper after the final barrier coat is all that is necessary to prep the boat for paint. 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 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.

Dealing with old paint

However, 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. Check the bottom paint compatibility chart below to see if the paint you plan to use is compatible with what’s on your boat now. 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 stripper system or other material on hand.

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 rinse with water. 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 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.

Cleaning a dirty hull

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.

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. Now get to work!

First, lay out a large tarp or dropcloth to cover the entire work area. Knock off any loose paint with a scraper and, using either a sanding block or a dustless DA sander, lightly sand the entire bottom with 80-grit sandpaper. Spend a little extra time on any high or uneven areas. Try to achieve as smooth of a surface as possible to minimize underwater drag. Work around transducers, prop shaft struts and outdrives. When you have finished sanding, wipe down the surface with a rag and some solvent wash.

Masking off borders

Next, select a masking tape that’s right for the job. Some things to keep in mind when selecting a tape: 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! Select a “long-mask” tape for bottom painting, especially when you’ll be applying multiple coats. 3M’s 2090 Scotch Blue Painter’s Tape is a good choice, because it's UV-stable and can be left on for up to 60 days. Try to tape as close to the existing bootstripe as possible. Start from a natural break point in the line and draw the tape every 1.5' to 2' for a smooth, even border. Be sure to tape around transducers, prop shaft struts, through-hulls and any other metal parts, as the copper in the paint will react with the other metals and cause galvanic corrosion. 3M’s Fineline tape is the best choice for when you are ready to re-paint the bootstripe (or for any other painting task where you want the cleanest possible paint line).

Bottom Painting

Applying the paint

You’re now ready to start painting. Select the right paint accessories to match the type of paint you are applying. Spraying is not recommended for do-it-yourselfers as it involves serious toxic hazards. 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. (We’ve tried this and can speak from messy experience here!)

Thin-film paints such as VC-17m are too runny for such a heavy nap 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 (most bottom paints are applied unthinned) or by spreading it 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.

Have the paint shaken just prior to application. This will make stirring the copper back into solution much easier, because it will settle into a thick mass at the bottom of the can, making stirring a real chore. After thoroughly stirring the paint, pour into the paint tray and roll the paint evenly from one end of the boat to the other.

A word about timing

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. Copolymer ablative paints have minimum overcoating times, and no maximum out-of-water time. Copolymers are 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 jackstands 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”.

Final Thoughts

Completing each of the key steps for painting your boat’s bottom is easy with the right knowledge base. We hope this Advisor and our companion video have helped answer your questions and helped you find the best products to do the job right the first time. Finally, for next year, consider this thought: spring weather can be unpredictable, and haul-out schedules in the yard are crowded. At season’s end, when you haul out for the winter, paint the hull bottom so you’ll be ready to launch in the spring, whatever the weather. Note that this only applies if you use a multi-season ablative type paint. Good luck!