Painting Aluminum

Question:

We are a small ornamental iron shop.In an effort to offer a more durable outside railing to customers, we have been manufacturing our ornamental iron designs from aluminum using traditional structural, just as with wrought iron, one-inch-square tubes, one-half-inch punched channels for framing, one-half-inch square solids for balusters, molded top rail or a channel for a top cover. These are all mill-finish bar stock from our supplier. We occasionally use spear points, picket castings or caps that are generally sand-cast aluminum. All items are of welded construction, with the visible welds ground smooth, and are then taken to be painted. We don’t use any type of pressure wash cleaning or blast cleaning prior to priming.

We have had a slow learning process. First using water-based primers from the local hardware stores, topcoated with oil-based enamels, we had a high rate of peeling especially on the large flat areas. We have progressed to a vinyl wash primer from different manufacturers some that are a two-part kit, with a quart of an acid, added per gallon of primer. We still finish with an oil-based enamel. The new process has been better. However, we are still getting occasional failures and peeling, primarily with the primer peeling.

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We want to offer a better product and still remain competitive, while powder coating has helped us with large jobs, it just isn’t cost effective for smaller jobs, custom colors, etc., and one of selling points is the fact they we will do small jobs, and try to offer more flexibility in design and colors for our customers. D.F.

Answer:

I commend your efforts to provide a more durable outside railing to your customers. One problem with iron railing is rusting, and that is a product failure. Although aluminum is more durable because it does not rust, it develops “white corrosion,” and that is a product failure. Paint peeling from an otherwise durable aluminum railing is also a product failure.

The simple solution to your problem is to provide proper surface preparation prior to painting. The last sentence of your first paragraph highlights the problem. While it is true that phosphoric activated vinyl wash primers are excellent primers for aluminum, it is also true that to be effective they must be applied to clean aluminum. Mill finished aluminum extrusions may have a contaminant acting as an interference coating on their surface. Finger grease from handling the parts is also a contaminant. The phosphoric acid must react with the aluminum to maximize adhesion. It will not work over oily soils and corrosion products. At the very least, you should power wash, to degrease your product, before applying the wash primer. Blast cleaning, after degreasing, would provide better surface preparation.