by Jim Seidel
article from Interlux website
Every year, boaters spend a lot of time and money applying antifouling paint to the bottoms of their boats for the season. Some boaters always seem to have better results than others. The reason for this may be the choice of antifouling paint but it may also be how the surface is prepared and how the paint is applied. Poorly prepared or applied bottom paint not only looks bad but it causes increased drag, poor fuel economy and may also lead to premature fouling.
Have you ever seen a hull bottom that resembles craters on the moon or dried mud flats? The roughness caused by chipping, cracking and loose paint increases drag, and can slow down your boat. These conditions may be a direct result of poor preparation, or simply too much paint build-up on the bottom. The yearly application of hard antifoulings results in the build-up of old spent coatings. As these coatings age the binders that hold the paint together, become weakened because of the water moving in and out of the paint film. This causes the paint to delaminate from the hull. Paint buildup can be avoided by sanding hard antifouling paints before recoating in the spring.
Hard antifouling paints work by leaching biocide out of the paint film and leaving the paint film behind, sanding removes the old, honeycombed paint film that is left on the boat. Power washing hard antifoulings will remove the surface contaminants; and paint will adhere to surfaces prepared this way; however power washing will not get rid of the old porous coating that is left on the hull and the roughness of the surface will remain. Sanding the surface with 60-80-grit sandpaper is the best way to smooth out and reduce the thickness of the old coating. Once sanding is complete wipe the surface with a rag that has been dampened with Special Thinner 216. This will help remove sanding residue and other contaminants that can affect the adhesion of the antifouling paint.
The use of copolymer or ablative types of antifouling paints, such as Micron Extra or Fiberglass Bottomkote Act, will also reduce the need for sanding and the build-up of old spent coats of paint. This is becoming the simplest way to minimize work while protecting your boat from fouling.
Cracked and chipped paint provides an excellent place for fouling to begin, -- especially weed and slime fouling. This type of fouling increases drag markedly by creating "mini-eddies" or turbulence between the boat and the water. This turbulence increases drag and costs fuel economy and speed. Weed and slime fouling can also be caused by skimping on the antifouling paint. Some boat
owners want a "real smooth" bottom so they will apply antifouling paint with a "real smooth" short napped roller. This type of roller may be good for topside coating but bottom paints are meant to be applied in heavier coats. To get the smoothest coating when rolling on the antifouling paint, use a 3/8" inch solvent resistant roller and "tip off" with a paint brush to remove the roller stipple. This method will result in a smooth finish and you will still have enough paint on the surface to provide adequate antifouling protection.
Remember the smoother the bottom paint the lower the resistance. The lower the surface resistance the faster the boat. This also translates to better fuel economy. There is only one way to ensure that the bottom is smooth --- and that is proper surface preparation. If in doubt sand it out!
Original article from Interlux website