Field Painting Unpainted Aluminum and Galvanized Steel

 In Maintenance & Repairs, Technical Information


Often customers need to know the correct way to paint a plain unpainted panel application in the field, such as the aluminum siding or roof. The following instructions should answer these questions using one of the procedures below:

A. Aluminum Exposed to the Elements for Less Than Six (6) Months

1. Prepare panels that need painting by cleaning with a common household laundry detergent. This would be a mild detergent such as Gain or Tide. After cleaning the panels, they should be rinsed completely with clean water. Panels should then be left to air dry.

2. Apply a cold water and phosphoric acid mixture to prepare the aluminum surfaces to accept and cling to paint. A pre-mixed solution of this type can be found in most paint stores under the name “Aluma-Prep” made by Amchem. The manufacturer’s instructions, found on the label, should be carefully followed when a commercial pre-treatment is used. A second option to buying the commercial pre-treatment is to prepare your own solution using the following ingredients and portions:

18% phosphoric acid (85 wt%)
22% water
25% isopropyl alcohol
35% butyl alcohol (by volume)

3. Apply the mixture above onto the aluminum panels with a cloth or brush. Do not allow the solution to dry. After approximately three to five minutes the panels should be rinsed off completely with clear water.

CAUTION: When using any acidic mixture, such as the one described above, precautions must be taken to shields skin, eyes, face, and any other exposed parts of the body. Be sure to wear protective clothing such as safety goggles and rubber gloves.

4. When aluminum panels are completely dry a coat of any good metal primer may be applied. (Do not use primer that contains lead pigments.) Aluminum paints that are made for metal are outstanding when used as primers.

5. Finish off with a coat of a quality exterior paint. The finish coat should be compatible with the primer that was used in Step #4. Confirm compatibility of the two with the paint provider.

B. Aluminum Exposed to the Elements for Six (6) Months or More

Use the same procedure as described in Section “A” excluding Step 2. (It is not necessary to pre-treat aluminum that has been out in the open after six months or more.) After Step 1 above, the aluminum will have a sufficient amount of self-etching to insure good paint adhesion. Continue on with Steps three through five.


Unpainted galvanized steel panels may need to be painted after being placed in the field. This may be done by using the steps below:

1. The panels may have accumulated “white rust” that will need to be removed before painting. If this rust is light it may be removed without too much loss of the zinc coating on the galvanized steel panel. Removing heavier white rust will probably need a more intense treatment which usually results in some loss of the galvanized coating. White rust is a type of oxidation and will involve some scraping to remove a heavy layer. The abrasion may be done by motorized equipment or chemical scouring.

Removing light rust from a galvanized steel panel before painting can usually be done by scrubbing the panel with household cleaner. Products such as Bon-Ami or Ajax will usually work on the white rust oxidation and the panel should be restored to the original finish of the galvanized coating.

As mentioned above, heavier white rust will need a stronger treatment to remove the oxidation. Of the numerous products that claim to do the job, two have actually been tested by a manufacturer of galvanized steel and discovered to be successful in removing heavier white rust residue.

One of the two products is called “ICC-258”. This cleaner is a powder that is mixed with water. The solution is directly spread over the area and allowed to stand for 30 seconds to 30 minutes. The time left on the white rust depends on the severity of oxidation. Before painting, the solution must be completely rinsed with water to prevent additional damage to the galvanize sheet.

The second of the two products is called “Deox Chemical Cleaner” which comes in a concentrated liquid form and is also mixed with water. This solution can be applied to the galvanized panel by several means. The panel may be immersed in the liquid mixture. Other methods include spraying, wiping or brushing the sheet with the chemical cleaner. Because this cleaner is very acidic, protective clothing including eye guards and rubber gloves should be worn. In addition, the area used for the cleaning treatment should have sufficient ventilation.

It should be noted that neither of these heavy cleansing products will entirely restore the brilliance or shine that the galvanized steel originally had. Both the “ICC-258” and “Deox” will predictably remove some of the zinc coating. Either cleaner will require a complete rinsing with warm water. After rinsing, the galvanized panels need to be fully dry before painting. Both of these products may be purchased from the following suppliers:

Intercontinental Chemical Corporation
2706 Gilbert Avenue
Cincinnati, Ohio 45202

Deox Chemical Cleaner
P.O. Box 2170
Irving TX 75060

National Chemsearch Corporation
P.O. Box 25
Monmouth Falls NJ 08852

National Chemsearch Corporation
P.O. Box 25
222 S. Central Av.
St. Louis MO 63105

Once the galvanized panels are clean and dry, they are ready to be painted, using the continuing steps following:

2. Using a primer and finish paint is recommended for best results. One coat of paint will cover the panels; however, applying two coats will give better results. The following details will be helpful for completing the job:

a. Primer Coat
A recommended primer for galvanized steel is called “metallic zinc”. These are available as Type 1 and Type 2, and either Type may be used. Application may be done with a sprayer, a brush or roller. Brand names of this primer include Sherwin-Williams Zinc Clad #5 or #7. These are zinc chromate primers. A local paint store should be of further assistance.

b. Finish Coat
The finish coat can be a second coat of the primer, or an actual finish coat in the color of choice may be used. In this case, the finish coat must be compatible with the primer coat. Again, the paint store should be able to assist.

The American Iron and Steel Institute have run tests that show new unpainted galvanized steel can be field painted immediately upon construction. It is not necessary to wait for the material to weather and no special treating of the galvanized surface is necessary prior to painting.

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