Coating Aluminum:
The inherent nature of an aluminum molecule is to react with oxygen immediately upon contact, combining to become aluminum oxide. Just like the iron in steel reacts to produce red iron oxide. Once the molecular surface oxidizes the process is effectively stopped, unless new metallic molecules resurface. Molecular resurfacing happens due to the existing oxide layer being removed by abrasion, acid, or erosion.
The longevity of any protective/cosmetic coating of the metal is only as good as the techniques & materials used to prep & apply the coating. That goes for paints, powder coating, & electrochemical materials/processes such as anodization.
As is the case for all final finishes, the principal & most important aspect is preparation. Without the proper prep work, all finishes fail....
Back to aluminum specifically:
Before any surface finish can be applied to aluminum, (other than anodization), the surface must go through a conversion process to change the oxide layer to a hydride layer, (AlH3). This done with an acid wash, usually containing dilute phosphoric or hydrochloric acid. Without this conversion any coated surface treatment is doomed to failure. The common chemical wash available is a product called Alumaprep, and is available through a number of manufacturers, including U.S. Paints which called their version AlumiPrep 33.
In simple analysis, this wash is creating, on a microscopic level, the same surface of aluminum hydride as is produced by anodization, which has surface layer thicknesses between .0002" - 0.1000". Most anodized product for the marine environment are between .0007" and .0012". So, this conversion wash leaves a very fine and fragile surface condition which is basically suitable for coating with other products.
To further increase the protective layer between surface coating and the aluminum, a process called alodining is called far. Alodine is a strong oxidizer and also does a chemical conversion of the surface. Alodine is applied in the same way as the acid wash, but leaves a thicker surface coating. Again, this surface condition is not nearly as thick as anodizing, but far greater than acid wash alone.
All paint or powder coat surface coatings are applied in the presence of oxygen & will trap some oxygen between the coating & the substrate, so great care must be taken not to disturb or scratch through these chemically generated surface treatments before the coating(s) are applied.
Painting: Not all paints are created equal... 2 part paints are superior to single part paints. Harder paints are more likely to resist abrasion than softer paints, but have more tendency to chip than softer paints. No paint, or even powder coat, should be applied without a suitable primer.
In most cases, for both paint and powder coat, an epoxy based primer is called for. Primers for paint are usually 2 part & primers for powder coating are single part.
There are specially formulated primers for use over metallic substrates that contain zinc. My favorite is a 2 part vinyl phenolic zinc primer,
(VPP-702), produced by
Preservo Paint Co. in Houston, Tx. This primer was developed for NASA &, until just a few years ago, was not available to the general public. Awlgrip's non-corrosive primers contain zinc.
Paint/powder coat vs. Anodized:
Cosmetically, the choice is up to the customer. Do you want the aluminum to be hidden & blend with the painted surfaces around it or do want the best & easiest way to surface treat the aluminum to ward off oxidation?
All three ways require periodic maintenance to remain effective barriers against oxygen reaching the aluminum. The easiest, but more frequently required, maintenance will be exhibited by anodization. But this maintenance must begin at the very beginning when the anodized aluminum is first installed. It must always be kept clean and free of salts and other minerals collecting on the surface. It must be sealed to block the porousity of the finish to keep corrosive substances from penetrating the hydrate layer.
While the hydrate layer of anodization is much harder than any paint or powder coat, it is ultra thin and can be scratched or abraded without much difficulty,
(Sorry, UV, but it ain't tough enough to stand up to a grinder!).
Again, once aluminum has access to oxygen, it's going to create aluminum oxide & spread like a cancer benieth the surrounding anodiztion. So maintenance must be preformed frequently. Never leave anodized aluminum un-washed after a trip or longer than a week or so. Always shammy dry wet anodized aluminum as soon as possible. All tap & sea water contains minerals. When water is left to dry by evaporation, the minerals are concentrated into the evaporating droplet and result in a very concentrated particle deposit. That pin point deposit will attack the anodization with a vengeance. Once the attack has successfully breached the hydride surface, it's game over, its a pit that grows larger & larger. Monthly, apply a sealant or wax to the entire surface. This both seals out moisture & enhances the beauty of the anodization.
On powder coating:
Like Jon C stated, "Powder coating has come a long way in the last 10 years". The advances are both product and process related. In terms of painted vs. powder coated metal, powder coating is far superior to painting. As always, the key ingredient in the equation is PREP!
Like any process, there are lots of those who can do the job, but only a few that can it right... In my little 3 traffic light town, there are two powder coaters. One makes steel gates, signs, & plasma cut sheet metal objects. They're good designers, good welders, and do a thriving business. They powder coat almost all of their products, but do so with a cheap gun, no primer, and labor that only understands the process in rudimentary terms. The other coater is highly profession, uses state of the art equipment & knows the process in depth. The difference is day vs. night... Both have done work for me, but only the second guy is trusted with something to go onto a boat.
In terms of powder coat chipping, I find that, as Robbie points out, one is not significantly better than another. The advantages that powder coat does have over paint, is that it's a heaver coating & reaches and protects into crevices far better than sprayed paint.
Bob, mentions problems with chipping around fasteners. This type of problem can be avoided by using a cushioning washer between the fastener & the powder coated part. Actually this is something that should be done even with a painted surface.
There are two factors in play here. One being the rotational force & friction of the fastener once it comes into contact with the coated part. The second is the torque pressure of the fastener bearing down on the coating. Even a standard metal washer between the fastener & the coating will reduce the problem, but best is a fiber or nylon washer that will absorb most of the destructive forces. TefGel & TufGel are great for isolating dissimilar metals but it's not designed for, or has, any cushioning abilities...
Br,
Patrick
