Direct Answer
Prepare steel for hot-dip galvanizing by: removing all weld spatter (grinding required), ensuring all welds are continuous (no skip welds on enclosed sections — gas trapping causes explosion), removing all paint and marking materials not compatible with the acid pickling process, oversizing bolt holes by 1/16" to account for zinc buildup, and adding vent and drain holes in hollow sections per ASTM A385.
Full Explanation
Surface preparation requirements for galvanizing: (1) Weld spatter — must be removed by grinding. Spatter doesn't pickle properly and falls off in the bath, creating bare spots. (2) Paint or markings — standard shop primer is incompatible with the acid pickle. Any member to be galvanized cannot be primed. (3) Flux — weld flux and slag must be wire-brushed and ground out. (4) Oil and grease — removed by alkaline degreasing at the galvanizer (they do this as part of the process, but excessive contamination causes delays). (5) Venting — ASTM A385 provides vent hole size and spacing requirements for hollow members. The galvanizer should review your assembly drawings before fabrication begins.
What This Means for Your Shop
Coordinate with your galvanizer before fabrication begins. Walk through the assembly drawings with their technical team to identify all venting, drainage, and surface prep requirements specific to your project geometry. Surprises discovered at the galvanizer's plant (missing vent holes, skip welds on enclosed sections) result in returned material and schedule delays.
Common Mistakes
Using skip welds on plates welded to hollow sections or forming enclosed voids without vent holes. The trapped air and moisture converts to steam in the zinc bath at 830°F — enough pressure to cause a metal explosion at the kettle.
Recommended Process
Develop a galvanizing checklist for each assembly: venting (location, size per A385), drainage (lowest point holes), weld spatter removal, continuous welds on enclosed sections, hole sizing, and no incompatible markings. Have the galvanizer sign off on your assembly drawings before fabrication.