Tips for the maintenance of sailboat bottom finishes are presented. Sailboat bottom system come in four kinds. Gel coats are used for dinghies, epoxy for sportboats and small keelboats, ablative or hard antifouling paint are used for cruiser/racers or racing keelboats.
Everyone who hauls their boat for winter storage knows that sooner or later the question of what to do with the bottom will arise. I’ll demystify the various bottom-finish systems by simply stating that a well-prepared bottom will last longer and work better. Now, how much time and money are you willing to spend? You can do the work yourself if you have more time than money, or you can go to a “speed shop” and have a professional team do the dirty work. Either way, the payoff is more speed.
Although boats come in all sizes, the scope of bottom systems is relatively limited. There’s the shiny gelcoat bottom, found on most planing dinghies and some sportboats. Other sportboats and small keelboats are finished in epoxy. Then, for those who keep their boats in the water, there’s ablative or hard antifouling paint.
Anti-fouling paint comes in two types: ablative, which will slough off over the course of the season, and hard, non-ablative. Wet sanding is not recommended for ablative paints because much of the finish is removed during the wet-sanding process, making the surface less effective against growth. Ablative paints, however, usually require less cleaning because the paint sloughs off while the boat is underway, taking the slime and growth with it.
Many racers prefer a hard, non-ablative paint, which can be wet sanded and polished to a shiny finish. An epoxy bottom provides an excellent water barrier and in most systems is recommended underneath the anti-fouling bottom paint.
You can finish the bottom with epoxy alone, but it will not have anti-fouling properties. The advantages, however, are durability and color. An epoxy-coated bottom will last many seasons because it does not wear off, and it comes in white. In addition to aesthetic reasons, white is a good choice because growth is more visible on a white bottom.
Doing It Yourself
If you choose to do the bottom work yourself, you have several choices. The easiest thing is to simply roll or spray another coat of paint over the existing bottom. This works, but is usually not conducive to speed or durability. Power washing, which many yards do routinely upon haul-out, will remove growth and loose paint. If you make the arrangements beforehand, the yard can usually take the extra time needed to remove loose paint with the power wash.
Ideally, however, bottom preparation starts with a bare hull, either gelcoat or coated with an epoxy barrier from the mold. If the bottom of your boat already has paint on it, you’re going to have to remove it. Sanding the existing paint can be difficult, especially if it’s ablative paint that will rapidly clog sandpaper. Chemically stripping the bottom is probably your best bet. This may require several applications of stripper, and will require sanding afterward. Electric or dual-action sanders, with a 5- or 6-inch round pad, work best. Manufacturers typically recommend mechanically scuffing the surface before any bottom application. I recommend 60- to 80-grit sandpaper to achieve a solid bond between the raw bottom and epoxy barrier.
After scuffing the bottom, apply the epoxy barrier to the manufacturer’s specifications. They will specify a mill thickness that generally equates to three to five thickly applied coats. Keep in mind that after sanding you will have fewer coats than when you started, so the rule is to go with more rather than less.
For a final application of anti-fouling bottom paint, sanding the epoxy with 120 grit, followed by 220 grit, is fine. When reapplying an epoxy barrier, sanding the underlying layer with 80 to 100 grit is recommended. The epoxy barrier coat is very hard, and depending on the application process (rolled or sprayed) a coarser grit sandpaper may be required to remove all of the “orange peel” from the epoxy. The serious racer would block or board sand at this point to fair the bottom.
When sanding, one of the tricks of the trade is to wipe blue dye on the bottom before sanding. This makes the high spots and pinholes more visible. Another option is to spray a light coat of spray paint on the surface. (Any residue left after sanding can be wiped off with acetone.) Any holes you find should be filled with epoxy filler, resanded, and primed. High spots should be sanded down. You now have a nice epoxy barrier. It can be sanded smooth as a final finish, or the barrier can serve as a primer for a finish surface of white epoxy or anti-fouling paint.
The best way to apply a finish coat is to spray it. Rolling the bottom paint or epoxy is also fine, but will require more work when wet-sanding. Apply the finish coats to the manufacturer’s specifications; again, three to five coats. After it’s dry, you’ll need to sand again. If the finish was sprayed, start with 320 grit and finish with 400. If rolled, start with 220 and work your way to 400. For a shiny finish, we always wet-sand with a rubber sanding block, using a fore and aft motion, keeping the block parallel to the centerline and waterline.
Maintaining your bottom from this point will require only routine cleaning, whether bottom paint, epoxy, or gelcoat finish. Keep in mind that if you clean with anything more abrasive than what the bottom has been prepared to, you will negate all of the hard work that has been done. A good wet-sanded finish, whether epoxy or anti-fouling, is usually 400 to 600 grit. Gel-coated surfaces are usually 1,000 to 1,200 grit, so cleaning with anything more than a sponge with dish soap will scratch – I recommend white Scotchbrite pads for in-the-water cleaning. Remember to clean in a fore and aft direction. Wetsanding also will clean well, but each time you wet sand, you remove more paint.
Whatever your choice of bottom treatment, remember that a well-maintained finish will get you there faster.