Coatings for Food Can and Beverage Cans

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Q: I am building an aluminum aircraft and am using a milspec spray pack Zinc chromate coating for the insides of the structure. Is this type of coating better than the hardware store variety cold gal zinc spray packs that do not contain zinc chromate and what is the chromate part of zinc chromate? Also, has Alodine coating proven to provide better protection than zinc chromate that has a sacrificial anode effect? Does your standard zinc chromate coating etch into the surface?

A: Thank you for your e-mail. Please note that zinc chromate primers are very different from Alodine™. Alodine is one step in a multi-stage pretreatment process and it does not provide sacrificial protection to the aluminum substrate. Instead, it acts as a passivating inorganic thin coating over which a primer can be applied. In the same vein a zinc chromate primer does not provide sacrificial protection but its corrosion inhibiting properties retard the corrosion process.

The military aerospace industry in the U.S. uses MIL-P-23377 primer followed by MIL-C-83286 polyurethane topcoat as the standard coating system. On the inside unexposed surfaces only the Alodine and MIL-P-23377 are used.

You cannot compare the MIL-spec coatings with those that you purchase at a hardware store. The latter are intended for the consumer market and do not possess the corrosion resistance or durability that you can expect from the specification products.

Finally, the MIL-P-23377 primer does not etch into the substrate but performs as a conventional primer.

Q: Would you recommend a paint coating to prevent corrosion to an aluminum diamond deck finish (3003 aluminum?)? If left to the natural weather elements (not near salt air), would this surface naturally oxidize to form its own protective coating? On 6061-T6 aluminum, I would also ask the same questions as above.

A: Unfortunately, you did not tell me what the aluminum diamond decking will be used for. In general, you are probably better off leaving it as is rather than coating it. In a marine environment the aluminum will have a tendency to corrode in the form of small deep pits, but these might not affect the structural integrity of the decking. If you do want to coat the aluminum you will do well to prepare it much as you would your aerospace components. If your corrosion shop has a pretreatment process such as Alodine™, (which is the same as you would use on aircraft), you should use it on the aluminum. You can follow with an epoxy primer and polyurethane topcoat, but be aware that as you apply more coats of paint you will diminish the traction of the diamond pattern. If the decking is intended for people to walk on then you do not want to smooth a finish and you might want to sprinkle sand or glass beads into the coating to render it non-skid.

If you don't have the Alodine™ pretreatment then thoroughly scuff sand and abrade the aluminum to remove all traces of oil, grease and corrosion products. Apply the epoxy and polyurethane as described above.

I hope this will help you but if you have further questions please do not hesitate to write again.

Q: Can't I use any exterior paint for a top coat? Also how do you suggest cleaning the surface. Do you know where I can get acid etch primer?

A: This is the follow up for preparation and painting the aluminum house. Scrape and remove all loose paint. For the remaining paint sand smooth and feather the edges. Clean the entire surface with a heavy duty water reducible detergent such as Formula 409. Follow instructions for general cleaning and rinse with clean water. Apply the acid etch primer also following product instruction for mixing and application. Within 24 hours of primer application apply either an acrylic gloss latex paint or solvent borne maintenance enamel. The acid etch primer is available at most national brand paint stores or at an automotive paint store.

Q: I have an aluminum lamp post that is painted matt black and is now showing considerable wear, particularly on the top over the light. I would like to touch it up or re-coat it entirely. What is the easiest process? Is there a one step coating for aluminum, or must I use an undercoat?

A: About refinishing the aluminum lamp post, I just completed repainting the post in front of my home. Use a pad of fine steel wool to remove any accumulation of dust and dirt; follow with a wipe down to remove the residue. Mask off the glass; I use aluminum foil for this part. Now use an aerosol spray can of antique black enamel and apply a couple coats over the entire surface. This procedure will last for several years with excellent protection.

Q: What would a common procedure be to resurface a corrosion pitted marine motor outdrive (mercury-aluminum outdrive). I have heard that 2 part paints and epoxies have incredible adhesive properties as well as corrosion resistance, but what would be recommended in this application?

A: An epoxy primer followed by a polyurethane topcoat is still my number one choice, but the real issue is cleaning up the pits in the motor outdrive. Once the pits have been thoroughly cleaned, your friend will need to immediately apply the epoxy primer. By immediate, I mean within a few hours and not the following day. One method to clean the pits and prepare the surface is by abrasive blasting with glass beads. But your friend will probably need to take the drive to a ob shop to have this done. In the industrial world the drive would probably be pretreated with a chromate conversion coating, but in all likelihood your friend won't have access to this process, unless he/she is willing to take the drive to a job shop that has the capabilities of pretreating aluminum. (Many job shops can blast clean the surface with glass beads and pretreat in a conversion coating).

If your friend does not want to take the drive to a job shop, another pretreatment coating is a "wash primer" which is a very low viscosity, high VOC organic primer that contains phosphoric acid, a vinyl butyral resin, corrosion inhibitor and solvent. I'm hesitant to suggest the application of a "wash primer", because if it is not properly applied blistering of the coating system can result. Therefore, I'm left with the recommendation that he/she apply the epoxy primer directly to the clean surface. Remember, the degree of cleanliness will determine the success of the recoated surface.

Unfortunately there are no guarantees, because so much depends on the substrate and the surface preparation. Good luck.

Q: I have an aluminum lund cammo boat that is need of a paint job. The local paint stores here have no idea on what kind of paint I should use. Can you help? What type and where can I purchase the paint. I have both a conventional and an airless paint set up; can I use any of these to re-paint the boat?

A: The surface area of the boat is considered to be small in size, therefore, I would choose to use the conventional air spray equipment for paint application. First clean the aluminum surface to remove all dirt, old paint and oxide from the surface. Apply a coat of acid etch primer following the mixing and application instructions to the letter. Within 24 hours of applying the etch primer, apply the topcoat finish. This will be an automotive grade or marine grade polyurethane coating. Use a good quality product to protect all the prep time and elbow grease you invested prior to the topcoat application.

Coatings for Food and Beverage Cans