Tips for Painting Boats

Revised by BoatUS editors in April 2012

  • Applying bottom pain on fiberglass boatAlways follow manufacturer's instructions and safety precautions as to applying new paint and dealing with paint already on boat..
  • Spread plastic on the ground to collect and dispose of paint chips, sanding dust, and drips.
  • Never leave bare wood exposed, it will affect your finish.
  • Wipe down and sand the surface free of wax coating.
  • Previously unpainted hulls must be cleaned with a dewaxing solvent to ensure paint adhesion.
  • Optimal painting conditions are when the temperature is 50-80°F and relative humidity is below 65%.
  • Apply paint or varnish on a medium temperature, low humidity low wind day for optimum drying and no dripping.
  • Several thin coats are better than one thick coat.
  • Wear protective clothing, especially for eyes and hands

Applying Bottom Paint

Preparation required for a successful bottom paint job begins as soon as the hull clears the water.

Clean

Slime and growth are relatively easy to remove while the bottom is still wet, but let the stuff dry and you will have to chisel it off. Fortunately most boat yards pressure wash the bottom as soon as they haul the boat, and many will also knock off hard growth with a long-handled scraper.

Strip

The adhesion of the new paint is only as good as that of the paint under it, so watch for signs of adhesion failure. Anywhere the old paint is flaking or lifting, scrape exposed edges with a knife or small chisel. If the paint zips off, the bottom may need to be stripped of paint.

You may also have to strip the bottom if you are changing paint type. 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.

Using a 2" hook scraper is one stripping method with less potential for injury to both you and the planet, and this is often the easiest method as well. If you decide to use a chemical stripper, be sure it is one formulated for fiberglass; regular strippers will attack the gelcoat. Always follow closely manufacturers’ instructions and warnings.

Sand

When the old paint is in good condition, in general you need only sand it, wash it, tape the waterline, and roll on a fresh coat or two. A grinder loaded with 80-grit disks on a foam pad can quickly prepare a hull for recoating, but it can also chew through the paint and into the laminate in an instant. If you lack experience with this powerful tool; 80-grit paper in a random orbit sander or a finishing sander will do the same job with less risk to the hull. Do not use a belt sander; it is designed to make things flat and that is the effect it will have on your hull.

Many boat yards now prohibit normal power sanding because of the dust it generates. The solution is a shop-vac and a random-orbit sander with a vacuum hose connection. If you don't want to buy a new sander, slip a length of plastic hose over the dust bag mount on your old palm sander and tape the other end into the shop-vac hose. Either rig will capture most of the toxic dust sanding generates-but not all of it. Be sure to wear a tight-fitting respirator-not a paper mask-while sanding. Also wear earplugs to shut out the din of the sander and the vac; you'll save your hearing and find the work much less tiring.

Even if you aren't stripping the bottom, it is good practice to sand away most of the previous application. This avoids a thick build-up that will turn brittle and cause new paint to flake. A different color first coat provides a flag that signals when you have sanded enough.

Toxicity comes not just from the sand paper, but from the old paint, particularly as it becomes airborne. It’s difficult, even with good capturing equipment, to prevent this entirely. Thus extreme care must be taken to protect you, others and the environment.  Often this job may best be done by qualified professionals.

Bare Fiberglass

A hull that has not been previously painted has mold release wax on the fiberglass that will interfere with paint adhesion unless you remove it. Clean the hull surface thoroughly with dewaxing solvent and plenty of clean rags before you sand; otherwise sanding drags the wax into the scratches and it will be that much harder to remove.

Sand the dewaxed hull lightly with 80-grit paper before applying the first coat of paint-the flag coat-which should be a different color from the top coat(s).

Stir

Run the can in a paint shaker for at least 5 minutes to get even distribution of the ingredients. Let it run for 10 or 15 minutes if it's been on the shelf a while. In the absence of a shaker, pour half the paint into a mixing bucket so you can mix the remaining half vigorously without sloshing paint onto the ground. Keep dredging up the substance off the bottom of the can until the bottom feels clean to the touch of your paddle. Slowly stir in what you poured off until the paint is uniform in color and consistency. You may also want to carefully use a paint stirring paddle which you can insert into an electric drill. These are commonly found in most hardware stores and paint stores.

Roll

Roll the paint onto the hull using a short-nap roller cover. An extension for the handle will make painting the keel easier and keep you clear of the inevitable droplets the roller will sling. Wear sleeves and gloves to keep the paint off your skin.

Fill your paint tray with paint. Dip your roller, unload it on the tray slope, and roll it up and down on the hull, i.e. from waterline to keel. Work fast as many bottom paints dry quickly. Each time you refill the paint tray, first stir the paint in the can to keep the contents 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 lengthens the life of almost any bottom paint; copolymers benefit from 3 or 4 coats. No sanding or other prep is needed between coats. Save some paint for the areas under the stand or cradle pads. Many prefer to put 3 or more coats around the water line and a foot or more below that because this is normally the area where most wear occurs and most fouling occurs.

Let bottom paint dry at least overnight before you put masking tape on it to paint the bootstripe. Get hard bottom paint into the water within the time specified on the label.

Types of Paints and Other Issues

Because of environmental concerns, increasing regulations which vary with locality and continuous research and development by leading paint companies, this subject is evolving continuously.  For a thorough, helpful and recent discussion, see “Antifouling 101—A Comprehensive Guide by Interlux, written by Jim Seidel, Assistant Marketing Manager.  It was released in early 2012.  This may be found in this Antiouling 101 Guide by Interlux.
Our reference to this material should not be construed to indicate that BoatUS endorses any one brand of paint or methodology over another.



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