Article
Galvanized Steel Guide (2026): Coatings, G90 Sheet, Cutting, Welding & Nesting
Cutlistor Team6 min read
Introduction
Galvanized steel is carbon steel with a zinc coating that sacrificially protects against rust. Fabricators see it as pre-galvanized sheet (G90, G60) for ducts and panels, or as bare steel that goes to the galvanizer after welding. Both paths affect how you cut, nest, weld, and handle fumes on the shop floor.
Coated sheet still nests in two dimensions like any flat stock. Kerf-aware layouts reduce how much expensive coated area becomes skeleton scrap. This guide covers zinc coating basics, common designations, cutting and welding precautions, HVAC and outdoor applications, and when to buy pre-galvanized sheet vs galvanize after fabrication.
What Is Galvanized Steel?
Galvanized steel is mild steel coated with zinc to slow corrosion. Hot-dip galvanizing dips fabricated parts into molten zinc. Continuous galvanizing on mill lines produces pre-galvanized sheet and coil used for roofing, siding, ductwork, and outdoor enclosures.
The coating is sacrificial: zinc corrodes before the steel under it. Cut edges and weld areas expose bare steel and need touch-up paint or post-galvanizing to restore protection.
Zinc Coating Designations: G90, G60, and Hot-Dip
| Designation | Zinc weight (both sides) | Typical service | Common products |
|---|---|---|---|
| G60 | ~0.60 oz/ft² total | Moderate indoor / dry outdoor | Economy duct, indoor panels |
| G90 | ~0.90 oz/ft² total | Outdoor, HVAC, general fab | Duct, enclosures, framing accessories |
| G115 | Heavier pre-galvanized | Harsher exposure | Coastal-adjacent when spec allows |
| Hot-dip batch | Thick zinc after fab | Structural outdoor | Posts, frames, welded assemblies |
| Galvannealed | Zinc-iron alloy coat | Paintable sheet | Automotive-style painted parts |
Typical Galvanized Sheet Thicknesses
| Gauge / decimal | Approx. mm | Typical uses | Cutting notes |
|---|---|---|---|
| 26 ga (0.018 in) | 0.5 mm | Light duct liner, trim | Shear, laser on thin coil line |
| 24 ga (0.024 in) | 0.6 mm | Residential duct, panels | Plasma, laser, lockformer |
| 22 ga (0.030 in) | 0.8 mm | Commercial duct | Laser, plasma |
| 20 ga (0.036 in) | 0.9 mm | Enclosures, outdoor panels | Laser, plasma |
| 18 ga (0.048 in) | 1.2 mm | Heavy duct, brackets | Laser, plasma |
| 16 ga (0.060 in) | 1.5 mm | Structural accessories | Laser, plasma, waterjet |
Cutting Galvanized Steel Sheet
Laser, plasma, and shear cut pre-galvanized sheet daily. Heat from thermal cutting vaporizes zinc near the kerf and can leave bare edge steel. Plan touch-up on visible outdoor work and ventilate the cutting area.
Laser cutting
Laser cuts galvanized sheet cleanly on many job shop lines. Fume extraction is mandatory: zinc oxide fumes are harmful. Model kerf in nesting layouts so duct sections still fit flange allowances.
Plasma cutting
Common on thicker galvanized plate and HVAC plasma tables. Wider kerf than laser: update Cutlistor kerf when switching processes on the same part set.
Shear, nibbler, and slitters
Shearing produces less fume than thermal cutting but still exposes bare steel at the edge. Lockformers and roll formers in duct shops slit and form coil with minimal waste when coil width matches layout.
Welding, Fumes, and Coating Damage Near Heat
Welding galvanized steel vaporizes zinc at the weld zone and nearby coating. Shops need ventilation, respirators where required, and weld procedures that limit burn-through on thin duct gauge.
Heat from welding destroys zinc coating adjacent to the joint. Specs often require zinc-rich paint or galvanizing repair on welds for outdoor structural work. Pre-galvanized sheet assemblies that must stay corrosion-free in service sometimes use bolted connections instead of field welds.
- Fume extraction at laser, plasma, and weld stations is not optional
- Grind weld areas and apply zinc-rich paint when hot-dip is impractical
- Avoid breathing white zinc smoke: follow OSHA and local guidance
- Thin gauge warps under high heat: stitch weld and clamp
- Consider galvanize-after-fab for welded frames that must be fully coated
Pre-Galvanized Sheet vs Galvanize After Fabrication
| Approach | Best for | Advantages | Limitations |
|---|---|---|---|
| Pre-galvanized sheet (G90) | Duct, folded panels, punched parts | Fast, no dip logistics | Weld and cut edges need touch-up |
| Hot-dip after fab | Welded frames, posts, railings | Full coating coverage | Distortion risk, vent holes required |
| Paint over bare steel | Indoor cosmetic | Color choice | Less sacrificial protection than zinc |
| Galvannealed + paint | Automotive-style parts | Good paint adhesion | Not the same as heavy outdoor hot-dip |
HVAC, Outdoor, and General Fabrication Uses
HVAC ductwork historically consumed large volumes of G90 sheet. Outdoor electrical enclosures, light structural accessories, and agricultural buildings also specify galvanized sheet for cost-effective corrosion resistance without stainless pricing.
- Duct: nest flat patterns before lockforming; account for seam allowances in part sizes
- Roofing and siding: often coil-fed; nesting still applies to flat blanking operations
- Outdoor enclosures: combine nested panels with linear-cut angle frames
- Fasteners: use zinc-compatible hardware; avoid uncoated carbon steel bolts on wet service
Nesting Pre-Galvanized Sheet and Yield
| Layout issue | Result | Fix in nesting |
|---|---|---|
| No kerf gap | Parts overlap at machine | Set kerf to laser or plasma width |
| Random part order | Extra sheet opened | Run 2D nest before PO |
| Ignored remnant | Usable offcut discarded | Enter remnant as stock on next job |
| Mixed gauges on one nest | Setup errors at table | Nest per thickness separately |
How Cutlistor Helps With Galvanized Sheet
Free use is genuinely unmetered where it matters for cutting: change a dimension, re-nest, change it again, at no cost. The 3-per-day allowance covers PDF exports and new projects. Cut lists stop at 20 rows, imported CSV or XLSX files at 5 rows, and there is no persistence between visits.
Galvanized Steel vs Mild, Stainless, and Aluminum
Galvanized carbon steel sits between bare mild steel and stainless on corrosion cost. Aluminum avoids rust without zinc but costs more. Stainless suits food and marine where zinc is not acceptable. Bare mild steel plus paint competes on indoor work.
Frequently Asked Questions
What does G90 galvanized mean?
G90 indicates about 0.90 ounces of zinc per square foot total on both surfaces of the sheet, a common coating weight for outdoor and HVAC applications.
Can you weld galvanized steel?
Yes, with proper ventilation and weld procedure. Expect destroyed zinc near the weld and plan repair paint or post-galvanizing when corrosion protection must continue.
Are laser cutting fumes worse on galvanized sheet?
Thermal cutting releases zinc fumes. Use fume extraction and follow safety rules for your jurisdiction. This is a shop safety topic separate from nesting geometry.
Should I buy pre-galvanized sheet or galvanize after fab?
Pre-galvanized sheet for mostly cut-and-fold work with minimal welding. Hot-dip after fabrication for welded structures needing full zinc coverage on all surfaces.
Does nesting matter on G90 sheet?
Yes. Coated sheet costs more than bare mild steel. Kerf-aware 2D nesting reduces skeleton scrap and sheet count on duct and panel jobs.
Conclusion
Galvanized steel trades a zinc layer for years of corrosion resistance at lower cost than stainless. Fabricators must respect fume and coating damage at cuts and welds, then nest coated sheet carefully so G90 pricing does not become G90 waste.
For duct, enclosure, or outdoor panel work, build the flat pattern list, nest with kerf in Cutlistor before ordering sheet, and choose pre-galvanized stock or post-galvanizing based on how much welding the assembly requires.