Aluminum boats have a classic look and appeal that sets them apart from wooden or fiberglass craft. And, they require very specific painting techniques and materials, as well. While aluminum was used in limited marine applications before World War Two, the aluminum boat industry really took off in the post-war period of the 1950s when the huge aviation manufacturing base developed during wartime was seeking new markets. The lightweight and strength of aluminum made it a natural match for boating.
Other aspects of aluminum, however, can make it more of a challenge to paint than marine materials such as wood or fiberglass. Aluminum is in a constant state of surface oxidation. This chemical process interferes with the ability of paint to adhere to the surface. Standard paints, even standard epoxy, will tend to peel off an untreated aluminum boat. In order to ensure that the primer coat sticks, any portion of a boat which is bare aluminum should be chemically etched to provide some gripping surface for the primer.
In the past, various solutions including home-made acids have been used to make aluminum paint-ready. Zinc chromate was the original etching primer used on aluminum boats and aircraft. It is still available, however it is used mostly for industrial applications as it requires specific safety precautions when applying. In recent years, however, many other self-etching primers—standard and two-part epoxy—have become available at the consumer level for the do-it-yourselfer. These are commonly available in auto parts stores in spray cans and from marine supply outlets in quart and gallon liquids.
These paints perform dual purpose, both lightly etching the aluminum surface and serving as a primer coat. Very often, aluminum boat owners choose to use a self-etching primer as the base primer, then after the base coat has thoroughly dried, apply a second primer coat of any of the standard consumer-grade metal primers.
The basic prep and priming procedure for aluminum boats is straightforward. Sand all exposed bare metal and existing paint. Any existing paint that is still well-adhered to the surface need not be completely removed but should be roughened up by the sanding. Wash down the sanded surface with water and a general purpose detergent such as TSP (trisodium phosphate.) Allow the surface to dry. Wipe the surface down with denatured alcohol or lacquer thinner and allow to dry. Spray a thin coat of self-etching primer on the boat or brush it on from a can of liquid. Allow the surface to dry. Apply a second coat of standard metal primer.
After the second primer coat dries completely, your aluminum boat is primed for application of marine grade epoxy or enamel top coat in the color of your choice.