The Bottom Line (Bottom Paint)

Boat PaintingCredit: West Marine

Whether you've just bought a new boat or are lavishing care on a classic, a good bottom job is a wise investment of both time and money.

Often boat owners make an enormous effort to make sure that a half dozen coats of varnish glisten in the sunlight, and yet the paint that protects the underwater portion of their boat's hull is scarcely given any thought. This flip-flop of priorities can cause both short and long term problems. In the sooner rather than later category lies poor adhesion of antifouling paint, as well as a performance downturn from a rough, uneven surface. Much more sinister are the long term effects that water has on fiber reinforced plastic (FRP) construction, and that's why most pros feel that barrier coating and careful antifouling are so important.

When the time comes to tackle a bottom job, it's essential to look at both your wallet as well as the calendar before deciding what's to be done and who will be doing the work. It's also important to understand the scope of the project and the extent of your own expertise. Barrier coating the bottom and finishing it off with a careful application of antifouling paint requires more than a quick scuff with sandpaper and a slap-dash "one-coat cures all" approach to painting.

In some ways, bottom work is a little like a visit to the dentist. What's in store ranges from a tolerable cleaning to a painful root canal, the latter being equivalent to what's entailed in curing a full blown case of osmotic blister, while the former is an easy-to- accomplish coat of bottom paint applied to an underbody that's in like-new shape. To a large extent, the scope of your commitment goes hand-in- hand with the condition of the gelcoat below the waterline and the antifouling paint’s condition. The following hands-on tips should give you a feel for what it takes to cope with a wide range of boat bottom blues.

The focus of the following guidelines targets mainstream bottom work, ranging from relatively straightforward bottom prep and painting to barrier coating and minor blister repairs. The heavy-duty task of fixing a badly blistered bottom requires more than simply rolling your sleeves up a little higher. It's a labor-intensive process in which care must be taken in order not to damage the fairness of the hull, trap water inside the laminate or place the proverbial band aid on a serious injury. So, before you commit to coping with the craters of the moon, call in an experienced surveyor or skilled boatyard fiberglass technician and get a pro's slant on the problem and your range of options.

Several Options

Option one, normally the most streamlined and straightforward, is the annual spring antifouling ritual, a fact of life for boaters who don't opt for a multi-year paint. Though initially more expensive, the cost effectiveness of superior paints are obvious when you look at their multi-season performance, ability to be relaunched and no-buildup, self-polishing nature.

If the bottom is in good shape and the paint shows no sign of chipping, flaking or crazing, all it takes is a light sanding prior to applying a coat of antifoulant. It's the simplest answer to bottom work but it's only appropriate for a boat bottom in good shape with little or no sign of paint or gelcoat problems. Using this approach on a boat plagued with deeper problems can be worse than simply postponing repairs.

The second alternative is still a feasible do-it-yourself project that's as appropriate for a brand new boat as it is for one with 10 years of paint peeling off the bottom. The process is a combination of barrier coating with a barrier coat product, followed by one or more coats of good bottom paint. Direct overcoating with antifouling paint can be carried out if care is taken and a smooth surface maintained during the application of multiple coats. Be sure to follow the specific time guidelines indicating when no additional sanding is needed prior to antifouling.


Once your boat has been hauled, pressure sprayed, and blocked, it's time to carefully check the bottom for blisters and circle problem areas with a contrasting color felt tip marker. This inspection needs to be done immediately because as the moisture evaporates from the surface minor blisters tend to disappear and even larger ones become harder to see.

  1. Reinspect the bottom after it is dry and use a knife or chisel to open up random blisters that appear on the surface or have been previously marked. Often, after the surface is dry, the oozing of a pinckish substance indicates blisters that you didn’t see earlier. Carefully determine whether these are paint blisters or deeper problems that penetrate the gelcoat. Tapping of the hull, if you know how, can also indicate blisters that don't show otherwise. If the surface contains thousands of blisters originating beneath the gelcoat, it's time to call in the experts. In extreme cases, some laminate removal and relamination of a new FRP layer(s) may be necessary. Spot repairs of badly blistered bottoms tend to be labor intensive and reblistering is not uncommon. Assuming that the blistering you encounter falls into the minimal to moderate realm, it's usually reasonable to continue to assume that a do-it-yourself approach is still feasible.

    Remove old bottom paint via sanding, paint remover/scraping and or stripping product (for fiberglass) or hire the yard to sandblast away the paint build up. It takes a pro to remove the paint and not chew up the gelcoat, but when such talent is available it's usually well worth the expense. Each of the of the paint removal processes has specific merits and disadvantages, and you should decide on a course of action with skill, time and dollars in mind. Sanding requires careful work with soft pad sanders and abrasives. The difference between removing the paint and cutting through the gelcoat can be a mere few thousandths of an inch. Those inexperienced with a sander can do quite a bit of damage to the surface that they are trying to improve. If you're unfamiliar with the idiosyncrasies of a high r.p.m. disk sanders you may be better off with a less aggressive orbital unit, chemical stripper, or contracting a sandblaster. The secret to mastering the art of soft pad sanding lies in keeping the sponge pad flat and continuously moving the machine while maintaining a light uniform pressure. The trigger switch should not be squeezed until the pad is on the surface and you are moving the sander as if it were on. Holding the 9" pad sander at shoulder height or overhead for hours at a time is like a lengthy visit to your local gym—make sure that your arms, shoulder, and back are up to the challenge. Powder residue from antifouling paint contains some nasty chemicals and it's important to wear an appropriate mask, eye protection, and clothing or coveralls that keep the dust off your skin. Keep in mind that your goal is to return the bottom to an unpainted scuffed-up gelcoat state with as few "burn through" spots as possible. Compressed air from a compressor that does not introduce oil to the air is the best means of removing the dusty residue prior to taking a close look at what should be roughed up gelcoat not a dished-out craters-of-the- moon kind of landscape.
  2. Wash the bottom one or more times (water only) if you notice chemical residue around any opened up blisters. Repeat the wash until the residue no longer forms. If chemical stripper has been used to remove the paint, clean the surface with a manufacturer recommended solvent wash. Chemical residue that's left on the surface can be a cause of future premature paint failure. These extra preparatory efforts tend to pay off down the road. Those in the northeast may want to do the prep work in the fall and allow the hull to dry over the winter, tackling the painting phase in the spring. The wham-bam all done in a week approach can lead to moisture being trapped by the barrier coat. If the problem is bad enough, you may need to have your boat “dry out” in a controlled climate. If no blisters are involved, allow the hull skin to dry out prior to tackling the application process. To be sure not to seal moisture into the laminate, you may want to hire a professional surveyor with a moisture meter. It can take many months for a hull to dry out. To test the hull for moisture yourself, tape small squares of clean plastic to several areas on the bottom. If moisture beads up inside, the laminate is too moist to coat.
  3. Tape off the bottom edge of the boot stripe with heavy-duty 3M long-term tape (A-02828 for a week or A-61174 for a month or two worth of exposure). Make sure all the application tools are on hand and brushes as well as roller pads are resistant to epoxy and any reducer you may use. The right material for the first coat is contingent on the condition of the surface being coated. For example, if numerous blisters have been opened, cleaned and flushed and the gelcoat has been sanded through in many areas, the bottom really needs an epoxy resin sealer coat. By sticking with one manufacturer's line of paint products, you're more likely to have intercoat compatibility as well as how-to guidelines, bulletins, and technical advice that will carry you through your project from start to finish. Follow the product manufacturer's instructions as to application.
  4. Follow the manufacturer’s recommendation for film thickness. One of the easiest ways to do this is to apply the amount of paint prescribed for a given surface area.

Tricks of the Trade

Epoxy resin (1000/1001) is not as easy to apply as a product specifically formulated to serve as a barrier coat, and it takes a bit of practice to nail down a technique that affords an even, sag-free surface. In this case, if too much material is applied to a near vertical surface, the paint sags and creates sanding problems. If too little is applied, the results are often marred by residual brush and roller marks and can yield too thin a skin to be an effective barrier coat. Reaching the happy medium is what skillful paint application is all about.

Tools of the trade include several different sizes of good quality natural bristle brushes, a rugged heavy-duty 7-9” roller frame and a supply of solvent-resistant rollers. A roller extension rod and a good sturdy pan round out a bottom painter’s tool kit. If possible, it also helps to set up a piece of plywood on sawhorses to act as a paint mixing workbench. This gets the chemistry above ground level and lets you better organize your mix, stir, and strain routine. When mixing any two-part paint, use some inexpensive kitchen measuring cups to get the volumes accurate. Glass or metal cups should be used because some plastic materials can be damaged by the solvents in the paint.

A key issue in the barrier coating process is building up adequate barrier thickness through multiple coats of paint. Refer to the manufacturer's guidelines to determine how long a window of time is available for direct overcoating without resanding the surface. Keep in mind that additional coats can't cure the sags, runs and other surface problems caused by poorly applied prior coats. If such problems arise on the first go-round, let it cure and sand out the imperfections before proceeding. With a careful hand, multiple coats can be built up without sanding in between.

Use the roll and tip technique to apply antifouling paint. Many experienced boaters, especially those plying warm saltwater estuaries and bays, swear by the slogan "don't skimp on bottom paint". The comment refers to both the amount and quality of the material applied to a boat's bottom. In this portion of the paint market, a boater certainly gets what he pays for, and the performance of top-of-the line paints show their value in less need for bottom cleaning during the season and much less prep work when it comes time to repaint. This can be a real value for sailboat owners as well as those with displacement powerboats. Faster boats tend to keep slime and soft growth from accumulating. The multi-year, relaunchable no build up copolymer ablative paints have year after year proven to be a very reliable product.

Boatyard Business

Whether you intend to do the work yourself or favor the "call me when she's ready" approach, a good working relationship with your boatyard staff is vital. This starts with a clear understanding of who will do each aspect of the work and just what restrictions the yard has on projects done by owners. Be sure that, if a subcontractor is to be involved, both the boat owner and the yard staff are on the same page, especially with regard to how billing will be accomplished.

There are also a variety of safety issues ranging from who is to move jackstands around in order to paint under pad spots, to what to do with waste solvent and paint cans. It's also a good idea to pay close heed to the time constraints enumerated in the haul out agreement; the job you planned to conclude in a week can stretch out when the rain clouds roll in. It's helpful to have a cushion built into the arrangement you have with the yard, just in case the weather or other variable is less than cooperative.

When making arrangements with a boatyard, look for cost effective options, not simply the lowest price. Take into consideration the condition of a yard's boat hauling equipment, security and the quality of the work that's accomplished by the staff. Do-it-yourself yards are getting harder and harder to find, especially when it comes to bottom work, but with a bit of willingness to range a little further a field, one can usually be found.

See also:

Antifouling Paints

Antifouling 101

Painting Tips


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