Applying Bottom Paint
by Don Casey
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 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.
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 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 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.
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 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 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 zinc anodes unpainted.
If you are installing new zincs-a good idea-make certain you don't paint
over their mounting locations. Good electrical contact is essential
for zincs 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.
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