Coating mass is not just weight gain

Zinc flake coating performance is often discussed in terms of coating mass, dry film thickness and salt spray hours. Weight gain can help estimate coating mass, but the meaning depends on surface area calculation, part geometry and whether excess retained coating exists in recesses.

A coating mass target from a flat panel or simple fastener cannot be blindly transferred to every component. Drainage, surface roughness and route selection affect the result.

Number of coats and curing

Many zinc flake systems use multiple coats with curing between coats. The first coat builds base protection, while subsequent coats or topcoats may improve barrier properties, appearance, friction or corrosion performance.

Under-curing, overloading, poor draining or insufficient cooling can reduce performance. Curing must follow the coating system requirement and substrate limitation.

Salt spray interpretation

Salt spray performance depends on coating system, film integrity, edge coverage, recesses, topcoat and packaging. A part may perform differently from a flat coupon because geometry holds salt solution differently.

Customer approval should define endpoint, significant surfaces, sample condition, packaging condition before test and whether threads/recesses are included.

When DFT and mass disagree

DFT measurement may be difficult on small, curved or irregular parts. Weight gain may include retained coating in cavities. Both methods have limitations. For technical approval, measurement method should be defined before production.

In some cases, supplier TDS, customer standard and agreed sampling data together provide the best control basis.

Practical conclusion

Zinc flake performance should be controlled by a combination of specification, coating mass/DFT, curing, geometry review and customer validation—not by one number alone.

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.