Powder coating expert Rodger Talbert explains to users when chemical treatment, sand blasting, or both should be performed for effective pretreatment.

Question: We make parts for agricultural equipment. Some parts are small brackets, and some are very large parts with bolt assemblies, welds, and different metal thicknesses. We want to switch from liquid coatings to powder coatings, and we are classifying the different options for part preparation. Currently, we clean all parts in a three-stage cleaning machine and apply baking varnish. We have some problems with rust around the weld and the edges of the parts, and we need to do better in these areas. We want the powder to hold better, but we know that we need a good surface to prevent rust and get good adhesion. We talked with chemical suppliers and they all suggested a variety of treatment processes, including cleaning and possible zirconia treatment. We also know that some other companies produce similar products and are currently doing powder coating and using sandblasting operations. Should we do chemical treatment, should we explode or should we do both?

Answer: In order to remove rust, you need to solve many challenges. First, you use a lot of hot rolled steel (HRS), which contains a lot of organic impurities (such as rust inhibitors) and a lot of inorganic substances (such as oxide scale). Therefore, the dirt on your parts will challenge any simple cleaning and disposal methods. In addition, you will add some challenges when cutting, welding, and assembling parts. Your pretreatment method needs to solve all these challenges to better remove rust.

First, you need a good chemical system that can remove organic dirt and treat the surface. Acidic cleaners may help decompose oxide scale and solder slag. If you have some aluminum parts, zirconium makes sense. It is also useful because it does not produce the sludge commonly found in iron phosphate or zinc phosphate solutions, keeping the nozzle clean and reducing maintenance.

Chemical acid solutions have been used to remove scale, rust and other inorganic substances, but I do not strongly support this. Some solutions are strong acids and require fairly high heat to be effective, which is not a good thing in the system. The wear effect is better and no dangerous solution is needed. Of course, wear has its own challenges. If sanding by hand, it requires a lot of labor and is not an easy task. If you set up a manual pressure blasting system, you will have someone suitable for arduous work with potential health hazards in a closed environment. You can set up a well-functioning in-line shot blasting system. Keep in mind that the wheeled shot blasting system will wear out over time and requires maintenance to keep it in good working condition. Nevertheless, the wheel blasting system may be your best supplement to deal with rust and scale. In addition, cutting will produce laser scales on the edges, which can lead to adhesion failure, so the sandblasting system can also handle this situation.

So the answer is that you should consider doing it at the same time-a multi-stage cleaning machine and a sandblasting system. This is the most expensive method and requires a lot of space, but it can reliably treat all soil and remove rust.

Q: Our parts have some sharp edges in the assembly, and due to the light coverage in these areas, we have encountered some corrosion problems. We worked with spray operators to make them work harder in the coverage of these key areas, but because the coverage was too small, we still had some defective products, and we got some rewards due to corrosion. Can you provide any suggestions on how to better cover these areas?

Answer: The sharper edges of the product are difficult to cover because there is really no surface to cover. The coating flows to the edge, but leaves a very thin coating. Some coatings will cover better than others, but leave less film on the edges than the flat area of ​​the part. The options for better coverage are changing the powder, changing the surface of the part, or adding two coats.

Talk to your powder supplier to ensure that the product you are using has flow characteristics that provide the best coverage at the edges. This is not a perfect answer, but it can help. You can grind that sharp edge to create some radius and get better coverage because the radius increases the surface area that can be adequately coated. If this kind of labor sounds like a problem (usually), then you have to go for double coating. The second layer of coating is indeed the best way to confidently solve the problem of edge coverage. The first layer flows towards the edge and creates some radius. The second layer flows over that radius and gives you real coverage. If corrosion is a problem, as you described, I will use an epoxy primer on the first layer to provide some additional moisture resistance.

Question: We produce outdoor gates and other railing products. Our railing products will change color after installation and are powder-coated. Most of these metallic colors are silver or gold. The finish has become a darker color, and after a while, the area that people usually touch when walking along the installation space is wiped off. Should we look for different suppliers of these coatings?

Answer: The metal powder contains aluminum or mica flakes to provide the eye-catching and shiny appearance that many consumers like. These coatings are generally unstable when exposed to sunlight, and they may also be sensitive to handling damage or abrasion. Most suppliers of these products, as well as powder coating research institutes and well-known experts in the industry, recommend applying varnish powder to the primer. Discuss this with your powder supplier and their selection of a suitable polyester varnish that is compatible with your metal color. To use varnish, you must first apply a primer and partially cure it (about 50-60%). Then varnish is applied and the two coats are cured together. Try some samples and be careful not to apply too thick a coating on the clear lacquer, so as not to lose the clarity of the base color. This will make your product perform better in the sun and resist contact damage. Please note that all organic paints, even with varnishes on them, will lose some of their color and gloss over time. However, after applying varnish, these finishes should have better results. The varnish will help protect the metal coating and provide you with an extra layer to extend the life of the finish.

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