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ARMORTHANE PROTECTING YOUR WORLD

PROTECTING YOUR WORLD

ARMORTHANE PROTECTING YOUR WORLD

Zinc Rich Primer:
Organic vs. Inorganic
& How to Spec It

How zinc protects steel, organic vs. inorganic zinc vs. cold galvanizing, what SSPC Paint 20 and ASTM A780 require, and tools to choose and quantify.
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85%+
Zinc by weight in the dry film for the top zinc-rich class, SSPC Paint 20 Level 1
SSPC Paint 20
1/10–1/40
Rate zinc corrodes compared with steel, depending on the environment
American Galvanizers Association
1.5–2.3x
Life of zinc plus paint versus the two used separately, added together
AGA, duplex systems
4.0 mils
Maximum zinc-rich paint thickness allowed when repairing galvanizing
ASTM A780 via AGA

A zinc rich primer is paint loaded with so much metallic zinc dust that the zinc particles touch each other and the steel. That turns the film into a sacrificial anode: where the coating is scratched, the zinc corrodes instead of the steel, the same way galvanizing protects a guardrail. It is the primer we see specified on bridges, tanks and structural steel when the owner wants decades, not years. It is also the most demanding primer to apply. This guide explains how zinc-rich primers work, the difference between organic and inorganic zinc and cold galvanizing compounds, what the SSPC and ASTM numbers on the can mean, and gives you a selector and a coverage and film thickness calculator that includes the ASTM A780 rules for galvanizing touch-up.

Quick answer

Use an organic (epoxy) zinc-rich primer for most shop and field work on blasted steel; it is more forgiving and recoats quickly. Use an inorganic (ethyl silicate) zinc for the longest life on near-white blasted steel, if you can control humidity during cure and mist coat before topcoating. Use a single-component cold galvanizing compound for galvanizing touch-up and small repairs. Zinc-rich primers need clean, bare steel to work; over rust or old paint the zinc cannot reach the steel.

What's in this guide
  1. 1How zinc rich primer protects steel
  2. 2Organic vs. inorganic vs. cold galvanizing
  3. 3Zinc primer selector (tool)
  4. 4Reading the specs: SSPC Paint 20 and ASTM A780
  5. 5Coverage and DFT calculator (tool)
  6. 6Application rules that matter
  7. 7FAQ
The mechanism

How zinc rich primer protects steel

Most primers are barriers. They work by keeping water and oxygen off the steel, and when they are scratched, rust starts at the scratch and creeps under the film. Zinc-rich primers add a second mechanism. Zinc sits above steel in the galvanic series, so when the two are in electrical contact with moisture present, the zinc corrodes and the steel is protected. The AGA puts zinc's corrosion rate at roughly 1/10 to 1/40 that of steel, which is why a few mils of zinc can protect for a long time.

That only works if three conditions hold:

  • Enough zinc. The particles have to touch each other to carry current. That is why the standards specify zinc as a percentage of the dry film by weight, and why a primer with a little zinc for color is not a zinc-rich primer.
  • Bare, clean steel. Zinc protects the steel it touches. Over rust, mill scale or old paint, there is no electrical contact and you have an expensive barrier coat. Most zinc-rich data sheets call for a commercial or near-white blast, with a profile.
  • A topcoat for long life. Zinc-rich films are porous and the zinc slowly gets used up. Topcoated, the zinc is held in reserve for scratches and edges. The AGA reports that zinc plus paint lasts 1.5 to 2.3 times longer than the two lifetimes added together.
Where this mattersThe classic failure on painted steel is undercutting: a stone chip or scratch lets rust start, and it spreads sideways under good paint. A zinc-rich primer stops that creep at the scratch. On galvanized steel, the AGA notes that bare spots up to about 1/4 inch across stay protected until the surrounding zinc is consumed.
Product types

Organic vs. inorganic zinc vs. cold galvanizing

SSPC Paint 20 splits zinc-rich coatings into Type I, inorganic (silicate and similar binders that cure by chemical reaction), and Type II, organic (epoxies, urethanes, vinyls and similar). Cold galvanizing compounds are single-component zinc-rich paints, usually organic. The rows below are real products from published data sheets. Sort by zinc content, film thickness or recoat time.

Product (type)Zinc in dry filmMin. prepDFT per coat (mils)Coverage at that DFTMin. recoatWatch-outs
Sherwin-Williams Zinc Clad 4100 (organic epoxy, SSPC 20 Type II Level 1)89.2%SSPC-SP 6, 2 mil profile3.0–5.0237–396 sq ft/gal30 min at 77°FTopcoat with listed epoxies and urethanes
PPG SigmaZinc 158 (inorganic ethyl silicate)Not stated on PDSISO Sa 2½ (near-white), 40–70 µm profile3.0–4.0348 sq ft/gal at 3 mils12 hr at 68°FNeeds over 50% RH to cure; mist coat before topcoat
Benjamin Moore Corotech V170 (organic, thermoplastic)83%SSPC-SP 3 (SP 11 suggested)1.7–2.3300–400 sq ft/gal12 hr at 77°FTie coat under some finishes; not over old coatings
ZRC Cold Galvanizing Compound (single component)95%SP 1 to SP 10 by condition1.5400 sq ft/gal at 1.5 mils12 hr; topcoat after 24–48 hrNo alkyd, alkyd-acrylic or lacquer topcoats
Sources: Sherwin-Williams Zinc Clad 4100 PDS; PPG SigmaZinc 158 PDS; Benjamin Moore Corotech V170 TDS; ZRC Worldwide ZRC data sheet. ZRC also reports passing 3,000 hours of ASTM B117 salt spray, a manufacturer test.
Selector

Which zinc rich primer fits your job?

Pick the job, the prep you can achieve, the humidity you will have during cure and what goes on top. The selector applies the data sheet requirements above and tells you when zinc is not worth it.

Zinc selector

Organic, inorganic, cold galv, or none?

Inorganic silicate zincs need near-white blast and over 50% humidity during cure (PPG). Organic epoxy zincs want at least a commercial blast (Sherwin-Williams). Galvanizing repair follows ASTM A780. Over rust or old paint, zinc cannot make contact with the steel.

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Recommended
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Prep check
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Topcoat note

Standards

Reading the specs: SSPC Paint 20 and ASTM A780

Two documents explain most of the numbers you will see on a zinc-rich primer can or in a project spec.

SSPC Paint 20: zinc levels

SSPC Paint 20 sets three levels by the weight of zinc in the dried film. Level 1 is 85% or more, Level 2 is at least 77% and under 85%, and Level 3 is at least 65% and under 77%. A spec that says "SSPC Paint 20, Type II, Level 1" is asking for an organic zinc with at least 85% zinc, which is what Zinc Clad 4100 at 89.2% meets. Type I inorganic products are further split by how they cure: I-A needs heat or a curing solution, I-B is water-reducible and cures by reaction with the steel and carbon dioxide, and I-C is solvent-based and needs moisture from the air to cure. That last one is why humidity matters with ethyl silicate zincs.

ASTM A780: repairing galvanizing

When galvanized steel is welded, cut or damaged, ASTM A780 is the standard practice for repair, and zinc-rich paint is one of the allowed methods. As the AGA summarizes it, the paint must contain either 65 to 69% or more than 92% metallic zinc by weight in the dry film. Prep is SSPC-SP 10 near-white for immersion and SSPC-SP 11 for less aggressive field conditions, and the repair must be painted before visible oxides form. The repair film must be 50% thicker than the surrounding galvanizing, but no more than 4.0 mils. The calculator below does that math for you.

Zinc percentage tricksRead whether a zinc figure is by weight in the dry film, by weight in the wet can, or by volume. Only dry-film weight compares to the standards. A can that says "contains zinc" or gives only a wet-can percentage tells you little.
Calculator

Zinc rich primer coverage and film thickness

Choose new work or a galvanizing repair. For new work, set your target dry film. For a repair, enter the measured thickness of the surrounding galvanizing and the tool applies the ASTM A780 rule. Coverage is theoretical: 1,604 sq ft per gallon at 1 mil, times volume solids, divided by DFT, using each data sheet's figures. Add your own allowance for spray loss and rough surfaces.

Coverage + DFT

How much zinc primer, at what thickness?

Volume solids implied by each sheet: Zinc Clad 4100 74%, SigmaZinc 158 65%, ZRC about 37% (400 sq ft/gal at 1.5 mils), V170 about 43% (300 to 400 sq ft/gal at 1.7 to 2.3 mils). Coats = target DFT / maximum DFT per coat, rounded up.

Repair mode only. Measure with a dry film gauge
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Target dry film
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Material, theoretical
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Coats / wet film per coat

In the field

Zinc rich primer application rules that matter

  1. Blast it if you possibly can

    Zinc Clad 4100 asks for at least SSPC-SP 6 with a 2 mil profile; SigmaZinc 158 asks for near-white with a 40 to 70 micron profile. Zinc needs to touch steel. Power tool to bare metal (SP 11) is the field fallback, mainly for organic zincs and touch-up.

  2. Keep the zinc in suspension

    Zinc dust is heavy and settles fast. Mix thoroughly and keep agitating in the pot. A film with the zinc left in the bottom of the pail is not a zinc-rich film.

  3. Hit the thickness, not more

    Too thin and there is not enough zinc to last; too thick and inorganic zincs especially can mud-crack. Check wet film as you go and dry film afterward. For galvanizing repair, stay under the 4.0 mil cap.

  4. Respect the cure

    Inorganic ethyl silicate zincs need moisture from the air to cure; PPG asks for over 50% relative humidity during curing. Organic zincs cure like other two-part epoxies and can recoat in as little as 30 minutes at 77°F.

  5. Mist coat before topcoating porous zinc

    Topcoats applied straight onto porous zinc can bubble as air escapes. PPG's sheet requires a mist coat followed by a full coat. Some organic zincs also need a tie coat under certain finishes.

  6. Pick a compatible topcoat

    Zinc is alkaline. ZRC's data sheet says not to topcoat with alkyd, alkyd-modified acrylic or lacquer products; use acrylic, epoxy, urethane or vinyl types. That is the same saponification issue that rules alkyds out on galvanizing.

Zinc-rich primer is one piece of a steel protection system. For the rest of it, see our metal primer guide, the metal coating guide and the corrosion prevention page. On steel that is already rusty and cannot be blasted, zinc is the wrong tool; look at rust-tolerant paint systems instead.

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FAQ

Zinc rich primer questions

What is zinc rich primer used for?

It is the first coat on steel that needs long-term corrosion protection: bridges, structural steel, tanks, offshore and industrial equipment, and for touching up damaged galvanizing. The zinc protects the steel sacrificially, so scratches and edges do not undercut the way they do with barrier primers.

Is zinc rich primer the same as cold galvanizing?

Cold galvanizing compounds are a kind of zinc-rich paint, usually single-component and very high in zinc (ZRC lists 95% in the dry film). They are not the same as hot-dip galvanizing, which metallurgically bonds a much thicker zinc layer to the steel. Cold galvanizing is mainly for repairs, welds and small parts.

What is the difference between organic and inorganic zinc primer?

Organic zinc primers use an organic binder such as epoxy; they tolerate less-than-perfect prep, recoat quickly and topcoat easily. Inorganic zinc primers use a silicate binder; they give excellent long-term and heat resistance but need near-white blast, humidity to cure and a mist coat before topcoating.

Can you put zinc rich primer over rust?

Not usefully. The zinc has to be in electrical contact with bare steel to protect it. Over rust, mill scale or old paint, it acts only as a barrier. Remove rust to bare metal first, by blasting or power tool cleaning to SSPC-SP 11.

Does zinc rich primer need a topcoat?

Some can be left alone in mild exposure, and a few data sheets say so. For long life and appearance, topcoat it. The topcoat slows consumption of the zinc, and the combination lasts longer than either alone. Use epoxy, urethane, acrylic or vinyl topcoats, not alkyds.

How thick should zinc rich primer be?

Follow the product. The examples here run 1.5 mils for a cold galvanizing compound, 1.7 to 2.3 mils for a single-component organic, 3 to 4 mils for an inorganic silicate and 3 to 5 mils for an epoxy zinc. For galvanizing repair under ASTM A780, it must be 50% thicker than the surrounding galvanizing but no more than 4.0 mils.

Can I paint galvanized repairs with spray zinc from the hardware store?

Only if the product's dry film zinc content meets ASTM A780, which requires 65 to 69% or over 92% zinc by weight in the dry film, and you prep to SSPC-SP 11 or better. Many bright "galvanizing" sprays are mostly aluminum or low in zinc. Check the data sheet, not the can's front label.

Can zinc rich primer go under polyurea or a bedliner?

Only if the liner manufacturer approves that primer and the procedure, including any mist or tie coat. Porous zinc can cause bubbling under a fast-set topcoat. For ArmorThane systems, applicators use the specified ArmorPrime primer; ask us about zinc under a liner for a particular job.

AT
Written by the ArmorThane technical team. We have manufactured polyurea, polyurethane and polyaspartic coatings in Springfield, Missouri since 1989 and train certified applicators to prep, prime and coat steel for industrial and transportation service. Third-party product data comes from each maker's published data sheet; your specification and product label govern. Updated October 2026.

Sources

  1. SSPC Paint 20 zinc-rich coating types and levels (summary): rustbullet.com.au (SSPC Paint 20 summary)
  2. American Galvanizers Association, zinc-rich paint repair (ASTM A780): galvanizeit.org/inspection-course/repair/zinc-rich-paint
  3. American Galvanizers Association, corrosion protection: galvanizeit.org (corrosion protection)
  4. American Galvanizers Association, duplex systems: galvanizeit.org/corrosion/corrosion-protection/duplex-systems
  5. Sherwin-Williams, Zinc Clad 4100 product data sheet: paintdocs.com (Zinc Clad 4100 PDS)
  6. PPG, SigmaZinc 158 product data sheet: dam-cdn.ppg.com (SigmaZinc 158 PDS)
  7. Benjamin Moore, Corotech V170 organic zinc rich primer TDS: media.benjaminmoore.com (V170 TDS)
  8. ZRC Worldwide, ZRC Cold Galvanizing Compound data sheet: zrcworldwide.com (ZRC data sheet)

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