Inconsistent Color Results With Clearcoating

We are experiencing difficulty in getting consistent color results when applying a 50-degree gloss clearcoat over various substrates. We use 10-gauge, 14-gauge and 16-gauge sheet steel, and 11-gauge steel tubing, all of which are hot-rolled, pickled and oiled. Our customer is asking that the finish be transparent to allow the welds to show through but not change the shade of the substrate. We are pretreating this material in our three-stage iron phosphate washer. After drying, we see various shades of blue to amber on the product. These colors are the normal result we get from this pretreatment.

However, after applying the clearcoat powder at 2.5 to 3.0 dry film mils and then curing at 400ºF for 12 minutes, we see various shades of amber. We tried underbaking, thinking the powder was overbaked, but found that although the finish looked closer to the natural look of the substrate, the gloss was 25 degrees too high. Does the chemistry in the iron phosphate need to be adjusted differently for clear coatings or is this a direct result of the iron phosphate process?

We asked many sources. Some suggested using an alkaline cleaner and rinse, followed by a nonchrome rinse and not using iron phosphate at all. Would this be a problem if the end product will be used in a store fixture in a somewhat controlled environment? Can you suggest a process that would give us some consistency with this project?

You may be correct regarding the work-hardening factor, but I also suspect the difference in color comes from the slight difference in steel alloy composition between the sheet and tube stock. In addition, the variance in phosphate spray impingement pressure can produce color differences. I doubt if any phosphate formulation can consistently give uniform color coatings because phosphate solution concentrations, temperatures and drainage factors all affect color. Talk with the chemical supplier about this, but I don't hold out much hope for you to be able to avoid color discrepancies.

A better avenue is to discuss the color expectations with the customer and the powder supplier. Almost nobody puts clear coatings over phosphated steel, and especially not when color is important. With the huge range of colors and appearance effects available in powder coatings today, this route has a much better chance of providing a desirable appearance that conforms with your customer's requirements.

On a product of this type, you do not want to skip the phosphate pretreatment. Iron phosphate is absolutely necessary to provide long-term adhesion of the powder coating. I worried when I read you played around a bit with bake temperatures. Underbaked powders are highly prone to chipping, so that must not be allowed to happen.

Powder Coating on Rock

I want to create thematic (art) objects that involve various irregular pieces of marble or granite and similar rocky materials. They will weigh anywhere from about 25 to 60 pounds each. The individual piece of stone is then creatively "wrapped" or surrounded with continuous or welded metal rod; both round and square rods are used. If I powder coat the metal rods, is it true that the powder will only coat the metal, since it is conductive, but not the rock, since it's not conductive?

No, I'm afraid it is not true. Some powder will likely get on the stone as well, even though the stone is not conductive. The cured powder coat on stone will be hard to remove. What can be done, however, is to carefully vacuum off the powder from the stone before curing. You will need to devise a small vacuum tool to accomplish this. A second possible method would be to mask off the areas where powder is not wanted, using materials suitable for the heat exposure during curing.

Long-Lasting Paint

Years ago the paint on the lower part of the cowl on my plane (a Mooney) was turning brown because of the exhaust pipe heat. To repaint with the original paint was expensive, so I experimented with Rust-Oleum from the local hardware store ($7.00 a quart). It has lasted now for 18 years.

That's great. I usually do not allow brand names in this column since salespersons occasionally submit glowing product reports of dubious veracity. But your report was checked, and I found out for myself that it was true. Thanks for sending it in.