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November 4, 2026steel buildings

Do Metal Buildings Rust? Galvalume vs. Galvanized vs. Painted Panels in Spokane (2026)

Do Metal Buildings Rust? Galvalume vs. Galvanized vs. Painted Panels in Spokane (2026)

Short answer: a metal building can rust, but a properly specified one in Eastern Washington shouldn't for a very long time. Roof and wall panels are steel with a metallic coating (Galvalume or galvanized), usually with factory paint on top, and that coating keeps red rust away, even at scratches and cut edges. Most rust problems come from the wrong coating for the use, or from things sitting against the panels: road salt, manure, treated lumber, copper, piled snow and metal shavings. Here's what the standards and panel manufacturers say, and how we spec panels for Spokane-area shops and pole buildings.

The quick answers

  • Will metal roofing rust? The steel underneath can, but the zinc or aluminum-zinc coating protects it, and paint adds another barrier. Coating type, coating weight and environment decide how long that lasts
  • Will metal roofing rust if scratched? A scratch that only goes through the paint leaves the metallic coating protecting the steel. A scratch that reaches bare steel is protected by the zinc in the coating, as long as there's coating nearby. Larger damaged areas should be primed and touched up
  • Will metal siding rust? Siding is the same coated steel. The bottom few feet of a wall take the most abuse: snow, slush, fertilizer and dirt
  • Which coating is best? Painted Galvalume for most shops; painted galvanized for livestock confinement, per U.S. Steel

What the coatings are

Galvalume (55% aluminum-zinc, ASTM A792)

Galvalume is a hot-dip coating of about 55% aluminum plus zinc, specified under ASTM A792/A792M. The name is a BIEC International trademark; Steelscape sells it as ZINCALUME. Common outdoor coating weights:

  • AZ50: 0.50 oz of coating per sq ft (total, both sides). Nucor calls AZ50 the standard coating weight for painted sheet
  • AZ55: 0.55 oz per sq ft. Nucor calls AZ55 the standard for bare, unpainted (often acrylic-coated) sheet
  • AZ60: 0.60 oz per sq ft, for harsher environments
  • Thickness: GalvInfo notes AZ50 is about as thick as a G90 galvanized coating

Why it works: the aluminum-rich part of the coating acts as a barrier, and the zinc-rich areas provide galvanic (sacrificial) protection that limits rust staining at sheared edges and other exposed steel (GalvInfo Note 1.4). U.S. Steel reports that over more than 25 years of atmospheric exposure, Galvalume has proven at least two to four times more durable than G90 galvanized in rural, industrial and marine environments.

Galvanized (zinc, ASTM A653)

Galvanized sheet is coated with zinc under ASTM A653/A653M. The number is the minimum coating weight in ounces per sq ft, total for both sides:

  • G60: 0.60 oz per sq ft
  • G90: 0.90 oz per sq ft, or about 0.45 oz per side if applied evenly
  • More zinc, more life: McElroy Metal's technical bulletin says that, all else equal, a G90 coating lasts about 50 percent longer than G60 in the same environment

Paint systems: PVDF (Kynar 500) vs. SMP

Most panels on shops and pole buildings are painted over the metallic coating. The two common systems:

  • PVDF: a resin of about 70% polyvinylidene fluoride, sold as Kynar 500 or Hylar 5000. Sheffield Metals calls it the most resistant to sun, temperature, humidity and pollution
  • SMP (silicone-modified polyester): the common alternative. In Sheffield Metals' comparison it carries the same film adhesion coverage as PVDF, but it is expected to chalk and fade more over time, especially on roof panels

Warranties are where the difference shows. In Sheffield Metals' published comparison, both systems carry 40-year film adhesion and 30-year chalk and fade coverage, but the limits differ:

  • PVDF: fade no more than 5 Hunter delta-E units and chalk no worse than an 8 rating, on roofs and walls
  • SMP: fade up to 7 delta-E units and chalk down to a 6 rating on roof panels, and 5 delta-E and 8 on walls

Read the fine print on any warranty. McElroy Metal's PVDF warranty, for example, states that failure due to substrate corrosion is not a covered condition, and some bright colors carry shorter terms. Finish warranties also commonly exclude corrosive environments, standing water and condensation (Berridge's sample warranty lists all three).

What happens at scratches and cut edges

Every panel has cut edges and fastener holes. Steelscape's technical bulletin explains that zinc and aluminum-zinc coatings protect cut edges sacrificially, and that more metallic coating means more protection on flat surfaces and edges. Nucor states the aluminum-zinc coating protects against the spread of red rust "even at cut edges or with minor scratches."

A few details worth knowing:

  • Short term vs. long term: Cleveland-Cliffs notes that galvanized may show less red rust at cut edges than Galvalume in the first several years (under 10, depending on environment), but the zinc coating deteriorates faster at edges and on the surface after that
  • How panels get cut matters: MBCI says field cuts should be made with nibblers, snips or shears, not abrasive saws, grinders or torches, which leave rough, uncoated edges that rust
  • Swarf (metal shavings): MBCI warns that cutting debris left on a roof can cause permanent staining and, if enough piles up in one spot, rust through the panel. Cut on the ground, not on the roof, and sweep up drill shavings
  • Touch-up paint: AEP Span says scratches not obvious from 6 to 9 feet will likely blend in with weathering, and air-dried touch-up fades faster than factory paint. Prime scratches to raw steel with zinc-rich primer, then touch up sparingly with a paint pen, not aerosol

Eastern Washington conditions that eat panels

Road salt and magnesium chloride on vehicles

WSDOT applies liquid sodium chloride, calcium chloride or magnesium chloride to roads before storms, and solid salt to melt snow and ice already on the road. The Spokesman-Review reported in 2008 that Spokane County and the cities of Spokane and Spokane Valley rely on magnesium chloride. Whatever the chemical, that chloride slush rides into your shop on the truck. U.S. Steel's bulletin says salt water destroys the protective oxide films on aluminum and zinc. In a shop, that means:

  • Keep slush off the walls: let meltwater drain to the floor, not against wall panels and base trim
  • Rinse it off: wash salt off lower wall panels and door jambs
  • Raise the steel: a curb or stem wall keeps panels above the wet floor

Fertilizer, manure and livestock

U.S. Steel lists nitric acid in fertilizers and animal wastes, and ammonia, among the conditions that attack zinc and aluminum coatings. For animal confinement buildings (hogs, cattle and poultry), it says Galvalume corrodes faster than hot-dip galvanized, and it neither recommends nor warrants Galvalume there. Its recommendation is painted G90 galvanized with a zinc phosphate treatment, plus insulation, a vapor barrier and good ventilation.

Barn, stalls or fertilizer storage? Tell us before we order steel, so the coating matches the worst use the building will see.

Snow piled at the base

Ceco Building Systems' maintenance manual says snow or ice should not be allowed to accumulate at the base of metal wall panels, because it may permanently damage them. U.S. Steel adds that materials that hold moisture against the coating, like wet insulation, leaves or dirt, break down its protective films. Around a Spokane-area building:

  • Plan where snow goes: keep roof slides and plowed piles away from walls where you can
  • Mind the grade: keep soil, mulch and gravel below the bottom of the panels and base trim
  • Shovel with care: Ceco says not to use metal shovels that can scratch the paint or Galvalume

Condensation inside the building

Moist air condenses on cold steel. U.S. Steel's animal confinement bulletin explains how condensate absorbs corrosive gases and dust and attacks the coating, even through paint. Our metal building insulation guide covers the energy code and condensation in detail.

Dissimilar metals and treated lumber

Ceco's manual is blunt: never let metal building cladding touch, or take runoff from, copper, lead or graphite, or it will "rapidly deteriorate due to galvanic corrosion" and void the warranties. Common culprits:

  • Copper and lead: copper gutters, lightning rod cable, AC condensate and lead vent flashings
  • Iron debris and graphite: saw fines, weld spatter, even pencil marks (U.S. Steel)
  • Wrong fasteners: Ceco says not to use zinc-plated fasteners or carbon steel rivets on the roof
  • Concrete: Ceco says mortar or concrete will etch paint or Galvalume, and U.S. Steel says the coated steel shouldn't touch uncured concrete

Pole buildings: watch the treated wood

Pole buildings use treated posts and skirt boards. U.S. Steel lists copper-bearing chemicals from treated lumber as something to keep away from coated panels, and SteelRoofing.com says wet treated lumber shouldn't touch bare or prepainted Galvalume, or even drain onto it. The fix is separation wherever panels or trim would sit on treated wood. Comparing building systems? See our red iron vs. pole barn comparison.

Care tips that keep panels rust-free

  • Inspect after other trades: Ceco recommends checking for scratches, debris and scrap whenever other workers have been on the roof
  • Keep it clean: MBCI suggests water and mild detergent, never bleach, wire brushes or abrasives
  • No ponding or debris: U.S. Steel notes the Galvalume warranty is voided for bare Galvalume roof panels pitched under 1/4:12
  • Route drains away: Ceco says not to let AC or evaporative cooler water drain onto the roof

How Raven specs panels

  • We ask how you'll use the building: vehicles and deicers, livestock, fertilizer, wash bays and heat all change the right coating
  • Coating for the use: factory-painted Galvalume for typical shops and garages, and painted galvanized where manufacturers say Galvalume doesn't belong, like livestock confinement
  • Paint system: we'll show you whether a panel is PVDF or SMP and what its warranty actually covers
  • Details: shear cuts, swarf cleanup, separation from treated wood and dissimilar metals, panels kept off grade
  • Panel profile and snow: our PBR vs. R-panel vs. standing seam guide and snow load guide cover the rest of the roof

Raven isn't tied to one manufacturer. We source each building from whichever can deliver fastest, and check its coating and paint options against your use.

What an installed metal building costs

Raven's going rate for an installed pre-engineered metal building is about $60 to $75 per sq ft, and installed pole buildings run about the same. Electrical, plumbing and utility hookups are not included and are extra. Panel coating and paint upgrades are part of your quote. See also our metal building kit vs. installed guide and shop sizing guide.

Get a metal building quote

Call 509-350-2016 or request a free quote with your building size, location and how you'll use it. See our steel building services and pole building services.

Sources

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