Coating build-up changes assembly

Every coating adds material or changes surface condition. Threads, bores, dowel holes, sliding surfaces, seal faces and press-fit areas are sensitive to even small changes. Zinc flake and Xylan/PTFE coatings can create functional build-up that must be accounted for in design.

Customers should not approve a coating only by corrosion result and then reject it for fitment if no thread or bore requirement was provided at RFQ stage.

Masking is practical, not invisible

Masking can protect areas from coating, but it may leave transition lines, edge effects or small witness marks. Complex parts may require plugs, caps, tapes, fixtures or manual masking. The feasibility and cost depend on quantity and geometry.

Masking expectations should be shown on drawing or confirmed through marked samples.

Threaded components

External threads may gain coating thickness and internal drives may retain coating. Thread-forming screws and close-tolerance fasteners need special review. For zinc flake, recess and internal drive retention are known practical concerns.

Go/no-go gauge requirements after coating must be disclosed. Torque-tension or friction requirements should be validated with actual mating parts.

Approval samples reduce disputes

Sample approval should include actual assembly, gauge checks, coating thickness, masking line review and packaging. A photograph alone is not enough for fitment-sensitive jobs.

The best approval records mention surfaces coated, surfaces masked, allowed transition marks and measurement locations.

Practical conclusion

Masking and fitment are engineering decisions. They should be discussed before coating, not during rejection.

How buyers should use this guide in an RFQ

For a technically complete coating RFQ, the customer should provide the drawing, material, quantity, current surface condition, required coating system, thickness or coating-mass expectation, salt spray target, masking requirement and packaging expectation. For zinc flake, Xylan/PTFE and phosphating work, route selection cannot be separated from geometry, surface preparation and post-coating handling.

If the part has threads, internal drives, blind holes, seal faces, bores or close-tolerance assembly zones, those areas should be marked before sampling. Coating build-up, retained coating, masking witness marks and post-curing handling must be accepted or corrected during sample approval rather than after bulk production.

Common avoidable rejection causes

  • Approving corrosion performance without checking actual assembly, torque, thread fitment or bore clearance.
  • Using ordinary cardboard or recycled paper directly against zinc-based coatings during humid storage.
  • Bulk-packing Xylan/PTFE or zinc flake parts so that sharp edges and threads abrade the coating during transport.
  • Leaving freshly blasted steel exposed before coating, causing flash rust or surface contamination.
  • Treating salt spray hours as a universal field-life guarantee without considering storage, handling and exposure conditions.

Documentation and approval discipline

For controlled coating work, approval should include coating route, surface preparation method, number of coats, curing condition, measurement method, visual standard and packaging method. If the customer later changes part geometry, incoming condition, dispatch packing, storage duration or fitment requirement, the approved coating route should be revalidated before production continuation.