Stainless steel powder coating

The passive oxide layer that makes stainless corrosion-resistant also makes it hard to adhere to. Mechanical keying by blasting, plus a conversion coating designed for stainless, gives the powder something to grip. Coating also hides the material, so any spec that depends on visible stainless must be settled before finishing.

The commercial consequence is that stainless is priced on its own material line, not blended into a steel rate. Material grade, thickness, tolerance and finish are quoted separately from conversion, so a change of specification is visible in the price.

Ganesh quotes to the drawing first and to ISO 2768 general tolerances where the drawing is silent. For stainless powder coating, the practical starting points are in the table below. They are starting points, not a guarantee: the part geometry, thickness and fixturing decide what can actually be held, and the quotation states the tolerance offered.

Film thickness is specified and measured, not guessed: 60–80 µm is the usual architectural range and 80–120 µm is used where impact or corrosion exposure is higher. Stainless must be cut with high-pressure nitrogen to keep the edge oxide-free; that is the difference between a weldable edge and one that has to be pickled. The route is quoted as one sequence so the customer does not have to coordinate a cutter, a former, a welder and a coater.

Coating stainless simply to protect it is usually unnecessary and can create a maintenance liability if the coating chips. Specify coating for colour, reflectance or chemical barrier, not for corrosion.

A manufacturability review before quotation is the cheapest place to find this out. Send the drawing, material, finish and annual quantity and the review comes back in writing.

• Coloured architectural trim
• Low-glare panels
• Equipment covers
• Signage frames

Specify powder where you need a durable, single-coat finish with good impact resistance and a wide colour range. Specify wet paint where the part needs a very thin film, a metallic or special-effect finish, or a touch-up-friendly surface. Specify blasting plus a zinc-rich primer plus a topcoat where the service environment is C4 or C5 under ISO 12944 and the part cannot be re-coated in service. For stainless the practical constraint is contamination: carbon steel tooling, brushes and blast media transfer free iron, so stainless work is kept on dedicated tooling and its own handling. Stainless raises the stakes on the decision: tool wear, gas cost and slower forming mean the volume needed to justify a tool is higher than for CRCA, and the wrong early choice is expensive to reverse once a die has been cut.

State the powder type, the RAL or approved sample, the gloss level, the film thickness range, the masking requirement and the tests expected: adhesion to ISO 2409 or ISO 4624, and salt spray to ISO 9227 with the hours stated. Say what must remain conductive. A coating specification without masking and DFT is a colour request, not a specification.

DFT is measured on the production part, adhesion is checked on a sample, and colour is compared against the approved standard under a defined light source. Cure is verified against the powder supplier's window. Coating records sit with the batch, and the pretreatment chemistry is monitored per shift.

Cost splits into pretreatment, powder, masking, rack loading, cure and inspection. Pretreatment is priced by area and batch; powder by area and film thickness; masking by feature count. A heavy assembly can cost more to cure than a larger, thinner panel of the same area, because oven time follows mass. On stainless steel, material is quoted as its own line at the prevailing rate for the grade and thickness, so a change of specification is visible rather than hidden in a blended rate.