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Aluminum Boats and Tanks: To Paint Or Not
To Paint?
I am frequently asked: Should I paint my
aluminum fuel tank? What do I use to paint it? How do I do it?
These questions also relate to painting aluminum boats.
How do I paint my aluminum boat?
I see that question on many boating forums. The answer is
not that simple. Aluminum
tanks and boats have the same qualities and so require the same
procedures for painting.
Aluminum used in boats and tanks has to be
tough and durable and also be able to take a dent without
breaking. The grade of aluminum used in the marine environments is
usually in the 5000 and 6000 series. Tanks should be 5052, 5083
or 5086. The
reason, without getting into chemistry and metallurgy, is that
these aluminum series are very corrosion resistant and least
affected by salt water and other corrosives found in the marine
environment. At the same time they have the strength and
elasticity for use on boats.
Aluminum is one of the best metals for
marine use. Bare
aluminum forms an aluminum oxide coating on its surface that
creates a barrier and prevents the metal from corroding.
Wipe away the oxide, or even scratch it and the metal
will begin to corrode. However, the aluminum oxide coating is self-repairing.
If the metal is kept clean and dry the oxide will reform
and again protect the metal.
For this reason aluminum tanks and boats are generally
not painted. In fact aluminum tanks are almost never painted. It
is best to leave tanks bare.
Tanks are usually out of the weather in a protected
compartment, and painting the tank will not give it any more
protection than not painting it. The process for painting
aluminum is so complex that painting a fuel tank is simply not
worth the trouble.
On the other hand, aluminum boats can
become rather shabby looking after years of use so owners often
want to paint them and spruce them up.
Some boat manufacturers paint their aluminum boats to
make them more attractive and stylish. When done by a
professional in the factory these finishes can last for many
years. But if not applied correctly the paint will soon begin to
chip and peel and look worse than the bare aluminum. So, people
who want to do it themselves, or have a boat painted by a
professional shop, need to know just what process needs to be
used to get a quality, lasting finish that will look good and
last for many years.
It comes back to the oxide coating. Most
paint simply will not adhere to aluminum because of the oxide
coating. So the metal must be prepared to accept paint. This involves
five steps.
These are not necessarily in order.
- Cleaning
and removing dirt, paint, oils, grease, and anything else on
the metal.
- Chemically
removing the oxide.
- Priming
the surface with a coating that will accept the paint
- Several
rinses with fresh water
- Painting
with a compatible paint.
I am not going to go into this in detail
because I am not an expert on painting metal surfaces. But there
are many web sites that explain it well enough for the DIY boat
owner to do it.
The Metal Boat Society: Preparing Aluminum
For Coating: http://www.metalboatsociety.org/phpbb2/viewtopic.php?t=2193&highlight=painting+aluminum
The Metal Boat Society: Paint for Aluminum
Boat: http://www.metalboatsociety.org/phpbb2/viewtopic.php?t=362&highlight=painting+aluminum
Expert Witness: Painting Aluminum http://www.ronjoseph.com/painting-aluminum.htm
Expert Witness: Painting
Aluminum Aircraft, Outboard Motors, Decks, Houses, Etc.
http://www.ronjoseph.com/Q&A/B2001_painting_aluminum.htm
Michael Kasten: Aluminum For Boats: http://www.kastenmarine.com/aluminum.htm
World Fishing Network-Dean Franklin: Painting Aluminum Boat Hull: http://www.wfn.tv/asktheexperts/BoatExpert/question7.html
Finishing.Com : Painting Aluminum: http://www.finishing.com/1200-1399/1318.shtml
Paintings and Coatings Resource Center: http://www.paintcenter.org/rj/sep03i.cfm
Iboats.com Painting Aluminum: http://www.allboating.com/boat_paint/#painting_aluminum
Federal MIL spec on Painting Aluminum
MIL-DTL-5541F Chemical Conversion Coatings on Aluminum and
Aluminum Alloys. http://www.wbdg.org/ccb/FEDMIL/dtl5541f.pdf
So what about aluminum fuel tanks? Is aluminum a good, safe, long lasting material for tanks? .
In a research project conducted by Underwriters
Laboratories for the US Coast Guard they found that the average
life of an aluminum tank was ten years.
This can be extended considerably.
I have seen aluminum tanks 20 years old that are still in
good condition.
Aluminum is a very suitable material for
fuel tanks in boats, as long as they are installed correctly,
mounted properly, and inspected regularly.
The enemy of all metal tanks is water.
Any metal tank should be mounted so that moisture will
not collect on any surface.
One of the most common mistakes is mounting a metal tank
on a sheet of plywood, foam, rubber or some other material that
absorbs moisture. The
moisture dissolves the oxide and corrodes the bottom of the
tank. Ideally the
tank should have air circulating around it on all sides
including the top and bottom. To allow air under the tank
requires using brackets that raise the tank above the mounting
surface. This is by far the best way to mount an aluminum tank.
A common practice is to use a non-moisture absorbing material,
such as neoprene on which to sit the tank.
I, however, do not favor this.
I believe that moisture can still collect between the
neoprene and the tank, but this is a controversial subject and
others may disagree.
Another common mistake is foaming the tank
in. There are
Federal Regulations and American Boat and Yacht Council (ABYC)
standards that must be met to foam in a tank.
One of the most significant of these is that the foam
must bond tightly to the surfaces of the tank. This is to
prevent any moisture from collecting between the foam and the
tank wall. The
requirement is that the bond strength between the foam and the
tank must be greater than the sheer strength of the foam.
This means that the foam itself will break under stress
before the adhesive bond breaks.
In reality this is almost never achievable because the
builders rarely prepare the aluminum surface to accept the foam.
Foam does not bond well to aluminum oxide. Aluminum must be
prepared to accept foam in the same way as it is prepared for
paint, so the bond fails, moisture collects on the tank, and the
tank corrodes.
Another requirement is that the foam cannot
be the sole source of support for the tank. In other words, the
tank must be independently mounted.
On outboard powered boats this is rarely done because the
Federal Regulations for tanks do not apply to outboard boats. So
you see many outboard boats with foamed in tanks, and the tanks
fail long before the average life span. However, the ABYC
standard does apply to outboard boats; so on boats manufactured
to ABYC standards you rarely ever see foamed in tanks.
Another common error is burying the tank
under a sole or behind a bulkhead where it cannot be seen and
cannot be inspected. This
also makes it very difficult to remove and replace.
I have heard far too many horror stories from boat owners
about how they had to take the boat apart to replace the tank.
If a tank is mounted properly, easy to get to and
inspect, it will probably last 10 to 20 years without leaking
Last but not least, many aluminum tanks on
small boats are installed as low in the boat as possible,
usually in the bilge. On many boats built today the builders do
not put limber holes in the bulkheads to drain collected water
out of compartments. So
water collects under the tank, and while it is on the trailer,
or underway, the water sloshes around and wets the tank on the
bottom and sides. Often
the boat sits outside on the trailer over the winter, with no
cover, and rain and snow collect, causing the tank to stand in
water for a long time. The boats should be covered and the
trailer tilted up so water can drain to the stern.
The fuel tank compartment should have a drain so water
can drain back to the stern where there is a boat plug or a
bilge pump that gets rid of the water.
The whole point is to keep the bilge and the tank dry.
What
about Ethanol? Does it have an effect on aluminum tanks?
See Much Ado About Ethanol. http://newboatbuilders.com/docs/Ethanol.pdf
It
has been known for years that aluminum tanks can corrode from
the inside out due to alcohol in the fuel. If the tank is left
in storage for many months phase separation can occur, resulting
in separate layers of fuel, alcohol and water in the tank.
Acidic compounds form at the boundary between the water and
alcohol and this corrodes the tank. The acids cause pinholes to
develop in the bottom of the tank. Boats left in storage over
the winter with full tanks have been found in the spring with
empty tanks and full bilges. So care must be taken to properly
store your boat. Tanks should be stored empty, or if being
stored for only a few months, co-inhibitors and stabilizers can
be added to the fuel to prevent phase separation. If you store
the tank this way, it should be completely full to prevent
moisture from condensing in the tank. But the best way is to
store the boat with empty tanks to protect the aluminum tank
from inside corrosion.
The American Boat and Yacht Council (ABYC)
(http://www.abycinc.org)
publishes a standard for gasoline fuel systems:
H-24 Gasoline Fuel Systems.
This standard has specifications for fuel tanks including
type of material, thickness and various tests the tank must be
able to pass. This
standard includes the Federal Regulations for fuels systems as
well. (http://www.uscgboating.org/safety/boatbuilder/fuel_landing.htm).
The Canadian and Australian Standards are based on the
ABYC Standards and are very similar to the European standards
for fuel systems. The
standard also contains specific requirements for the
installation of the tank. ABYC also has a standard for Diesel
Fuel systems, H-33 Diesel Fuel Systems, which includes diesel
fuel tanks. Meeting these requirements can significantly
increase the life of the fuel tank and the safety of the boat.
Copyright February 12, 2008
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