PRE-GALVANIZED VS. HOT-DIP GALVANIZED STEEL: WHAT'S THE DIFFERENCE?
Both are carbon steel protected by zinc, and both are technically hot-dip processes. The difference is when the zinc goes on. Pre-galvanized steel is coated at the mill as a continuous coil, then cut and formed into parts. Hot-dip galvanizing after fabrication means building the finished part out of bare steel and then dipping the whole thing in a kettle of molten zinc.
That timing changes how thick the coating is, whether cut edges and welds are protected, what the part can look like, and what it costs. It's also why a galvanized duct and a galvanized handrail are two very different products.
Published October 6, 2026 · JMC Fabrication
Pre-galvanized sheet, sometimes called mill-galvanized or continuous galvanized, runs as a steel coil through a molten zinc bath at the mill. It's typically ordered to ASTM A653 with a coating designation like G60 or G90. Those numbers are the total zinc weight on both sides combined: 0.60 or 0.90 ounces per square foot. G90 works out to a little under 1 mil of zinc per side.
Because the coating is already on when the sheet arrives, the shop can shear, brake, and seam it without a separate finishing step. That's why almost all commercial HVAC duct is pre-galvanized sheet. It's the material SMACNA's duct standards are built around, and the project specification usually names the coating weight.
The tradeoff is at the edges and joints. Every shear cut exposes bare steel along the edge, and any weld burns the zinc off around it. Zinc does protect small exposed areas sacrificially, so a sheared edge on indoor duct is rarely a problem. Welds and heavily worked areas on exposed work need touch-up, usually with a zinc-rich paint.
Batch hot-dip galvanizing, covered by ASTM A123 for fabricated steel, happens after the part is completely built. The finished fabrication is cleaned, pickled, fluxed, and lowered into molten zinc at roughly 840°F. The zinc reacts with the steel and coats every surface it can reach: inside, outside, edges, and welds.
The coating is much heavier than mill galvanizing, commonly several mils thick depending on the steel's thickness and chemistry, and it's metallurgically bonded to the part. For structural steel, platforms, handrail, supports, and anything that lives outdoors, that's the version that lasts.
The catch is that the part has to be designed for the dip. It needs vent and drain holes, it has to fit the galvanizer's kettle, and thin or unbalanced sections can distort from the heat. Our guide to designing steel for hot-dip galvanizing covers those details.
PRE-GALVANIZED SHEET HOT-DIP AFTER FABRICATION When zinc goes on At the mill, on coil After the part is built Typical spec ASTM A653, G60 / G90 ASTM A123 Coating thickness Under 1 mil per side (G90) Several mils, all surfaces Cut edges, welds Exposed, touch-up as needed Fully coated Typical use HVAC duct, light sheet work Structural, outdoor steel Lead time impact None, coated on arrival Adds a trip to the galvanizer Design limits Normal sheet metal practice Venting, draining, kettle size
- Indoor HVAC duct — Pre-galvanized sheet, to the coating weight in the spec. Dipping duct after fabrication isn't practical and isn't what the duct standards assume.
- Outdoor or rooftop duct on the coast — Salt air along the Gulf eats zinc faster than an inland climate does, so G90 is the usual floor for exposed duct here. Where duct sees constant salt spray or washdown, stainless is often the better long-term answer than heavier galvanizing.
- Structural steel, platforms, handrail, and supports — Hot-dip galvanized after fabrication. These parts are welded, have lots of edges, and live outside, which is exactly where a mill coating falls short.
- Small brackets and hangers — Either can work. Pre-galvanized is cheaper and faster for light indoor parts. For anything structural or exposed, galvanizing after fabrication is the safer call.
If you searched for galvanizing in Gulfport, Biloxi, or Moss Point, it helps to know how the work is split. Pre-galvanized sheet arrives at a fabrication shop already coated from the mill. Hot-dip galvanizing after fabrication is done at a galvanizing plant with a zinc kettle, which is a separate facility from a fabrication shop. The fabricator's job is to build the part so it galvanizes well and to account for the extra step in the schedule.
Pre-galvanized and hot-dip galvanized steel both use zinc to protect carbon steel. Pre-galvanized is thinner, cheaper, and coated before fabrication, which makes it right for duct and light sheet work. Hot-dip galvanizing after fabrication is thicker, covers every edge and weld, and is the standard for structural and outdoor steel.
JMC Fabrication works in pre-galvanized sheet for SMACNA duct every day and builds carbon and structural steel detailed for hot-dip galvanizing, out of a 50,000 sq ft shop in Pascagoula, MS serving the Gulf Coast. If you're not sure which finish your project needs, send the drawings and the spec.
This article supports the following JMC capabilities: