Powder coating is one of the most popular finishes for stamped metal parts, but it behaves very differently from liquid paint. When a part is designed without accounting for how the powder adheres, cures, and builds up, the result shows up later in thin edges, blocked threads, trapped moisture, and parts that no longer fit together. The good news is that most of these problems can be avoided long before the press ever runs.
For anyone who buys or produces sheet metal parts, the finishing stage should not be an afterthought. Making a few smart decisions during designing sheet metal parts leads to a tougher finish, a shorter delivery time, and far less rework on the shop floor.
Why Finish Considerations Matter in Sheet Metal
Stamped parts are everywhere, from automotive body and seat structures to brackets and panels inside household appliances. In many of these products, powder coating is chosen because it is tougher than wet paint, and because it creates a smooth, uniform layer in a single application. It is also an environmentally friendly process, producing almost no volatile solvents.
But powder coating is a high-temperature process. Parts are sprayed with electrically charged powder and then cured in an oven at around 200 °C. The same features that make a stamped part strong and light also create the geometric challenges that can ruin a coating, which is why it pays to work with a partner that understands the full production route.
Design Rules That Prevent Coating Failures
Radius the sharp edges. During curing, the melted powder pulls away from sharp corners because of surface tension. The result is that edges end up with a coating that is far thinner than the surrounding surfaces, and these thin edges become the first place for rust to begin. Adding a small radius of around 0.5 to 1 mm to external corners lets the powder wrap evenly and keeps its thickness right at the point where chipping usually starts.
Watch the pocket depth. Charged powder prefers to land on exposed surfaces rather than deep inside narrow recesses. This is known as the Faraday cage effect, and it means tight U-channels and deep pockets may end up thin or bare at the bottom. As a rough rule, the depth of a pocket or channel should not exceed its width. If a deeper form is unavoidable, it is often enough to design it as two separate pieces that are welded or fastened later.
Allow for the coating thickness. Powder coating is not invisible. It typically adds between 0.05 and 0.15 mm to each surface, and because it coats both sides of a hole, the inside diameter of a bore shrinks by up to twice that amount. If sliding fits or mating holes are designed right at the nominal size, the coated part will not assemble. Designers should oversize openings and add clearance to tab-and-slot joints so that powder-coated sheet metal parts slide together smoothly instead of jamming.
Protect the threads. Cured powder is hard, and it will lock a bolt into a tapped hole if it is allowed to build up inside. The cheapest solution is to list the threaded holes that must be masked and to keep the holes at common sizes, so the coating line can use standard silicone plugs. Tapping powder out of a finished hole by hand takes around ten times longer than simply plugging it before the spray booth.
Preparing the Part for the Coating Line
Before any powder is sprayed, sheet metal parts go through a wash and chemical pretreatment. This stage is where closed shapes cause the most trouble. Welded tubes, folded box sections, and internal channels can trap cleaning liquid, and when that liquid boils inside the curing oven, the expanding gas blows holes in the coating, a defect called outgassing.
Adding small vent or drainage holes at the lowest point of the part lets the chemicals drain freely and prevents this problem. Similarly, a small hidden hanging hole is useful because every part on the line is hung on a grounded hook. The hook point leaves a tiny bare spot, so a dedicated hole in a non-cosmetic area is better than letting the operator guess where to clamp.
The substrate also matters. Mild carbon steel needs proper degreasing and a phosphate conversion coating before the powder will bond, while welded assemblies should have weld spatter ground down, because glossy powder magnifies rather than hides surface defects.
Choosing the Right Finish
Not all powders are the same. Epoxy powder is strong indoors but can chalk and fade under sunlight, so polyester is usually the better choice for exterior parts. Standard in-house colors cost less and are processed faster than custom shades, which may carry a minimum-order quantity. Where the operating environment is severe, it is worth specifying the required salt-spray hours on the drawing rather than only naming a color.
Working with the Right Manufacturing Partner
The design choices described above make the biggest difference when they are raised early, ideally with the company that will actually produce your powder coated sheet metal parts. A manufacturer with its own tooling, stamping, and finishing experience can flag a pocket that is too deep, a thread that should be masked, or a color that will add weeks to the schedule before the die is ever cut.
Established in 2003 and now supplying OEMs and Tier suppliers across more than ten countries, DIAN STAMPING is a factory-level producer of automotive and household-appliance stampings, tooling, and welded assemblies. With roughly two thousand sets of medium and small dies produced each year and prototype work available from drawings, 3D data, or physical samples, the company treats sheet metal part design as a shared responsibility between the designer and the manufacturer.
End-to-end capability means deeper pockets are re-routed as two-piece designs, threaded holes are masked before they reach the spray booth, and curing ovens are matched to the parts on the line. The result is a finish that survives both the assembly floor and years of service, at a price that only a factory can offer.
To see how your upcoming program can be designed for a durable, dependable powder-coat finish, reach out to the DIAN STAMPING team with your drawings or samples for a quote.