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Preparation required for a successful bottom 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 boatyards pressure wash the bottom as soon as they haul the boat, and many will also knock off hard growth with a long-handled scraper. If bits of bottom paint flake off under the pressure of the washer nozzle, ask the yard worker to make another pass to remove as much loose paint as possible.
Strip?
Remember that 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, worry exposed edges with a knife or small chisel. If the paint zips off, the bottom needs to be stripped.
You may also have to strip the bottom if you are changing the type of paint. 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-inch hook scraper is the stripping method least injurious 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.
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 somewhat less quickly but with much 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 boatyards 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 eventually turn brittle and cause new paint to flake. A different color first coat provides a flag that signals when you have sanded enough.
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 de-waxed 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
If the boatyard has a paint shaker, run it for at least 5 minutes to get the copper and the pigment evenly distributed throughout the paint. A drill-powered mixing paddle can also do a good job. In the absence of a either, 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 copper 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. If the paint has been on the shelf awhile, getting it mixed thoroughly can take 10 or 15 minutes, but don't skimp; if the copper isn't evenly distributed, some areas of your hull won't be protected.
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.
Don't add any thinner to bottom paint unless the manufacturer specifies otherwise. (There are exceptions to every rule: thinner may be required if the day is hot and windy.)
Fill the basin of 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 copper 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.
Get the yard manager to move the stands as soon as the rest of the hull is dry (never move stands yourself!), and put rags or sheet plastic on the pads to protect your new paint. Prep the bare spots and apply the appropriate number of coats. Save a little paint to slap on the areas on the bottom of the keel you can't get to until the boat is lifted.
Prop and Shaft
You can paint the prop if you like, but copper-based paint won't stay on a bronze prop (nor bronze rudders and struts) for long. Prop paints are available, but demanding prep — up to four prime coats — discourages their use. A heavy coat of wax on the prop will keep it clean for a time.
Don't paint the shaft, and be sure you leave all anodes unpainted. If you are installing new anodes — a good idea — make certain you don't paint over their mounting locations. Good electrical contact is essential for anodes to do their job.
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.
For more information about painting, consult Sailboat Refinishing by Don Casey