FT Archives

Roobol on painting

Color Differences After Pretreatment

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. Then, 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.

Faraday Cage and Rust on Fencing

We are a small job shop and powder coat fence panels on a regular basis. We have had a few instances where finished panels have been installed, and rust forms where the pickets pass through a piece of channel. Typically, the pickets are 1⁄2-inch square tubing, or solid, and they pass through a U-shaped channel that is an inch and a half across, with a 1⁄2-inch lip. The pickets are welded on the underneath side. We use a cleaner/rinse/phosphate/rinse process to prep the parts. The parts are dried in the oven, allowed to cool, and then powder coated with a TGIC polyester powder.

Although there is a Faraday cage effect in this area, I do not believe this is the cause of our problem. I think the issue is that where the pickets pass through the channel, it is so tight that they touch the channel on one or two sides. We cannot get our powder between the two pieces of metal, thereby leaving the two raw surfaces uncoated. Over time, rust begins to creep out of this area. We have recently tried spray-bomb primer on these areas before powder coating in hopes that some will make its way into this tight area, but have only had these panels in the field for a short time, so we are yet to see the results. It makes me nervous to be using a spray-bomb primer on anything around here. Do you have any suggestions on how we can get a good coating in these areas, or is it possible?

Actually, the Faraday cage causes the absence of coating and so is a problem here. To coat that area is indeed almost impossible once the welding is done. You can precoat with a weld-thru primer only on the areas that are to be welded, but the cost is likely to be rather high. The only other way to avoid rust that I can think of is to caulk the join area with a color-matching material. You can try the spray can primer, but don't hold out much long-term hope for corrosion resistance. It's hard to get coating in that gap with nonelectrostatic spray also, but at least there is no caging effect with which to contend.