Buyer guide
Prepared by LD Screw · Reviewed · Practical guidance for fastener RFQ preparation.
A finish changes more than color. Coating method and thickness can affect corrosion resistance, dimensions, friction, installation and compatibility with the base material. Name the coating process or governing requirement instead of requesting only silver, black or galvanized fasteners.
Common finish directions
Performance varies with coating system, thickness, sealing, handling and exposure. Confirm the project requirement rather than relying on a generic finish name.
| Finish direction | Common reason for selection | RFQ questions |
|---|---|---|
| Plain or lightly oiled | Indoor use, further processing or a specified uncoated condition | Oil requirement, storage protection and appearance acceptance |
| Electroplated zinc | General appearance and corrosion protection for many indoor or mild exposures | Color, passivation, coating requirement, thread fit and high-strength restrictions |
| Hot-dip galvanized | Heavier zinc coating for outdoor or project use | Coating standard, oversize nut or thread allowance, appearance and assembly fit |
| Black oxide / black finish | Dark appearance and limited protection when used with oil or sealer | Exact process, oil or sealer and corrosion expectation |
| Stainless / passivated condition | Corrosion resistance from the selected stainless material and surface condition | Stainless grade, passivation requirement, contamination control and environment |
Coating thickness and thread fit
Coating builds on the thread surface. A heavier finish can change assembly fit, especially on galvanized bolts and nuts. The bolt, nut and coating route should therefore be reviewed as one system.
High-strength fasteners may also require process restrictions or additional controls. State the property class before selecting an electrochemical coating process.
- Specify the coating method or standard, not only the color
- Confirm whether bolts and nuts are coated before or after thread sizing
- State any friction, torque or lubrication requirement
- Request samples or approval criteria when appearance is important
Choosing a coating for bolts and screws
The best fastener coating depends on the base material, strength level, exposure, thread fit and installation method. Use the application as the starting point, then confirm the exact coating specification with the supplier.
| Application direction | Common coating route | Details to confirm |
|---|---|---|
| Indoor or mild service | Electroplated zinc is a common starting point | Passivation color, coating thickness, appearance and thread fit |
| Outdoor construction or structural use | Hot-dip galvanizing or an approved zinc-flake system may be considered | Project standard, coating thickness, thread allowance and matching nuts and washers |
| Dark appearance for indoor parts | Black oxide or phosphate with oil or sealer | Exact process and realistic corrosion expectation |
| Humid, washdown or chloride-related exposure | A suitable stainless grade or engineered coating system | Exposure severity, crevices, mating metals and cleaning conditions |
| High-strength fasteners | A qualified process selected for the property class | Hydrogen-embrittlement controls, friction requirement, baking or testing when specified |
Sacrificial protection, salt-spray tests and service life
Zinc-based fastener coatings can protect steel sacrificially, but coating performance depends on the complete system: pretreatment, thickness, passivation or topcoat, part geometry, handling damage and the real environment.
A salt-spray result is a controlled comparison, not a direct promise of outdoor service life. If a project requires a test, state the test method, acceptance point and required hours together with the coating specification.
- Do not select a bolt coating from color alone
- Distinguish minimum coating thickness from a general finish name
- Confirm whether scratches, recesses and threaded areas have separate acceptance rules
- Include corrosion testing, friction or torque requirements in the RFQ when controlled
Finish information for quotation
Describe the operating environment and the required service life or project specification when known. A supplier cannot select a reliable finish from a product photo alone.
- Base material and mechanical property class
- Coating process, color, thickness or governing standard
- Indoor, outdoor, marine, chemical or high-temperature exposure
- Mating materials, thread fit and installation method
- Inspection, test report, packing and handling requirements