Mistakes in metal fabrication finishing are expensive. They may result in the customer complaining and wanting their order reworked or replaced, or more seriously, the structure or piece of equipment could fail prematurely, with implications for liability and reputation.
This blog looks at how to avoid finishing mistakes. After discussing why finishing is important and the finishing processes used on metal fabrications, we address the five most common mistakes and how to avoid them. We hope that after reading this, designers working on a fabrication project will come to us for advice on finishing.
Why Finishing is Important
Finishing a metal fabrication entails giving it the surface finish and appearance it needs, protecting it against corrosion, or both.
For some projects, surface roughness matters most. Fire truck bumpers are an example. These are highly polished to make them as reflective as possible. Less obviously, a smooth surface on fabrications used in food and beverage manufacturing aids in cleaning and stops bacteria from adhering.
Many fabrication customers want a high-quality appearance for their parts or assembly. This usually means no flaws or blemishes, and often, a particular color scheme.
Finishing is also about protecting against corrosion. Most steels will rust, and the only way to stop this is by covering the surface completely. Nonferrous metals can corrode too, especially if placed in a saltwater environment. Again, the right finishing process will protect the underlying metal and prolong the life of the part.
The Main Options for Finishing
Finishing processes are divided into: mechanical treatments, coatings, and conversion processes.
Mechanical treatments
These are buffing/polishing, brushing, and grit or bead blasting. Buffing and polishing are used to impart a smooth, usually reflective, appearance. Brushing gives metal a directional texture or grain-like appearance, and bead blasting produces a randomized texture.
Coatings
Paint is an option, but powder coating is a more widely used finishing process in fabrication. This entails electrostatically spraying a layer of dry epoxy powder granules onto a metal part, and then baking it in an oven.
The painting processes used are E-coat and conventional spraying. E-coat involves immersing the part in a tank of paint held in solution and using electrostatics to draw the paint onto the part. Spraying, using a spray gun and compressed air, is also often done with electrostatics to get a good coating that bonds to the metal surface.
Another coating method is electroplating. This deposits a metal like copper, nickel, or chrome over the top of a less expensive material like low-carbon steel.
Conversion Processes
These modify the surface of the part, either to change its appearance or to add corrosion resistance, hardness, or durability. The main conversion processes used in fabrication are: galvanizing, anodizing, and passivating.
Galvanizing entails dipping steel into a tank of hot zinc. The zinc forms a corrosion-resistant surface with a distinctive spangled appearance.
Anodizing is used mainly on aluminum. It’s a way of increasing the thickness of the surface oxide layer to improve corrosion resistance and raise hardness. It can also give the surface an attractive color.
Passivating is a chemical process used in fabrication to rid a stainless surface of carbon steel particles that would otherwise form rust spots.
5 Common Finishing Mistakes and How to Avoid Them
While finishing is rarely an expensive step in the production process, the cost of getting it wrong can be very high. This is because a poor quality or defective finish will, at a minimum, need to be removed and reapplied. In cases where reworking isn’t possible or practical, the alternative is to replace the part.
Fortunately, most finishing mistakes are easily avoided. Here, in reverse order, are the top five mistakes, and how to avoid them.
5. Skipping or insufficient Quality Control
When there’s pressure to hit a deadline or to save some money, businesses are sometimes tempted to reduce inspection. In a multipart order, that might lead to sampling, which could miss coating defects like runs, drips, or orange peel-type effects. On a single-piece order, rushing could result in subtle flaws not being seen, or perhaps being allowed through.
What’s the solution? First, don’t skip or rush the final inspection. Let the QC person do their job. And second, consider adding a QC check just before coating. By verifying the fabrication is clean and ready, coating defects and rework could be avoided,
4. Wrong finish for the material
This refers to specifying a finish that isn’t going to meet requirements. It could be that the finish doesn’t provide sufficient corrosion protection or is the wrong color or reflectivity. Other problems could be that the process specified is incompatible with the material, (you can’t anodize steel, for example), or will degrade the material in some way.
The solution is to work with the fabricator. Discuss what the design requires, (corrosion protection, a smooth surface finish, a particular color), and ask the fabricator what they recommend.
3. Insufficient curing
This occurs with powder coating. After powder application, the fabrication bakes in an oven while the epoxy particles fuse together. However, this is more than just a melting process. The polymer chains need time at temperature to become entangled, which is what gives this type of coating its strength.
The problem is that time is in short supply in manufacturing, and the coater often has other work waiting for the oven. This is why there can be pressure to rush this step of the process.
The solution? If your design requires powder coating, work with a vendor that prioritizes quality and doesn’t overbook their oven capacity. It may appear a little more expensive, but it’s almost certainly cheaper than having a coating fail prematurely.
2. Inconsistent application
Electrostatic processes use an electrical charge to attract particles onto the surface being coated. This improves consistency over line-of-sight processes like spraying without electrostatics. However, electrostatic charge concentrates at sharp points and corners, increasing coating thickness in those areas and leaving flat regions with a thinner layer.
The solution is to consider charge concentration and variable thicknesses during design. As far as possible, avoid creating points and sharp corners. If they are unavoidable, allow for increased coating thickness in these regions.
1. Poor preparation
Far and away the biggest reason for problems with coatings and conversion processes. Poor preparation results in poor adhesion or surface modification. Adhesion is a complicated subject, but it boils down to reducing the surface energy so the coating can form good bonds. (On a surface with high energy, water beads up into droplets.)
Reducing surface energy requires thorough cleaning. This should start with mechanical or abrasive methods, then follow up with chemical cleaning. One of the challenges is that not all contaminants are visible to the eye. Silicone, found in hand creams and cosmetics, can be a particular problem.
Solution: ask your fabricator how they clean parts before finishing. You might also ask how they check that surfaces are clean enough for coating. (The answer should be, that they perform a water break test.)
Ask us About Finishing Processes for Metal Fabrications
Finishing is an integral part of metal fabrication and should be considered during design. Most finishing processes are inexpensive, especially when compared with the value of the fabrication, but can be very costly if performed incorrectly.
In this blog, we’ve reviewed what can go wrong with the main processes, and how to avoid mistakes. The main takeaways are: consider finishing during fabrication design and talk to your fabricator about what you want and expect.
With decades of experience in metal fabrication, we’ve learned a lot about finishing processes, and we’re happy to share that with our customers. If you have a fabrication project where you could use some advice on finishing, or you just need a quote, contact us to start that discussion.