When a fabrication enters a zinc bath near 450°C, asymmetric sections, thickness transitions, residual stress and nonuniform heating/cooling can release as distortion. Heated air, moisture or acid in sealed cavities creates dangerous pressure; missing drains trap zinc, add weight and cause defects. Venting and draining are safety requirements, not optional cosmetic holes.
Values in this article support engineering screening during design review, RFQ and prototyping. They do not replace formal calculations, material data sheets, qualified procedures or contractual standards.
HOLE PLACEMENT
Place vents near the highest point and drains near the lowest point of every enclosed volume in actual hanging orientation, connecting each space divided by internal plates. End-capped tubes, sleeves, stiffeners and laps often create hidden cavities. Diameter and total area depend on volume, path length, viscosity and galvanizer handling. Map every cavity and hanging direction in the 3D model, then obtain galvanizer approval; one universal “10 mm hole” is unsafe.
REDUCE DISTORTION
Use symmetric sections and similar thicknesses; avoid broad overlapping surfaces with sparse welds; complete controlled welding/straightening before coating. Temporary stiffeners on long thin plates require galvanizer agreement on material and removal. ISO 1461:2022 gives general coating requirements for fabricated iron/steel, but hole layout, handling and distortion limits still require order/drawing agreement.
PROJECT CHECKLIST
All cavities; hanging direction; high/low points; baffle communication; trapped liquid; thickness transitions; distortion datums; threads/fits; repair.