Polyurethane
Coating: Types,
Specs & Systems

Q: What is polyurethane coating used for?

Weather-resistant topcoats on steel structures and equipment, waterproofing membranes on decks and below-grade concrete, roof coatings over spray foam, sprayed liners for trucks and equipment, floor topcoats, and spray foam insulation. Clear polyurethanes are also common wood finishes.

Q: Is polyurethane coating waterproof?

Yes. Elastomeric polyurethane membranes are used specifically for waterproofing, and products like Sikalastic-320 NS meet ASTM C836 for below-grade waterproofing. Thin decorative polyurethane topcoats are water-resistant but aren't designed as membranes.

Q: Can you put polyurethane coating on metal?

Yes, as part of a system. Steel is blasted (typically SSPC-SP6 minimum, SP10 for severe service), primed with zinc or epoxy, then topcoated with an aliphatic polyurethane at about 2–5 mils. In ISO 12944 C4–C5 environments, systems of roughly 240–300 µm total are typical.

Q: What's the difference between aliphatic and aromatic polyurethane?

Aromatic polyurethanes (based on MDI or TDI) are tougher per dollar but yellow and chalk in sunlight, so they're used as base coats, membranes and liners. Aliphatic polyurethanes (HDI, IPDI, H12MDI) hold color and gloss outdoors and are used as topcoats.

Q: What is a 2K polyurethane coating?

A two-component coating: an isocyanate hardener mixed with a polyol resin just before application. It cures by chemical reaction, typically has a pot life of 2–6 hours and outperforms single-component (1K) moisture-cure products in most industrial uses.

Q: How long does polyurethane coating last?

As part of a protective system on steel, ISO 12944 durability ranges run from under 7 years (Low) to over 25 years (Very High), depending on the environment and system. Topcoats are often refreshed during that life to restore gloss and color.

Q: How thick should a polyurethane coating be?

Topcoats on steel are typically 2–5 mils dry. Waterproofing membranes are 30–60 mils, and traffic deck systems around 35–40 mils. Always follow the product's data sheet.

Q: Is polyurethane coating toxic?

Uncured isocyanates are respiratory sensitizers, so spraying requires proper respiratory protection, ventilation and training. OSHA's ceiling for MDI is 0.02 ppm. Fully cured polyurethane is an inert plastic.

Q: Is polyurethane better than epoxy?

For different jobs. Epoxy is the better primer and adheres strongly to concrete and steel; aliphatic polyurethane is the better topcoat for UV, gloss retention and abrasion. Many systems use both: epoxy underneath, polyurethane on top.

Q: Why is my polyurethane coating bubbling?

Usually moisture. Isocyanates react with water to form CO2, which gets trapped as bubbles when applied in high humidity, over damp surfaces or too thick. Data sheets typically cap humidity at 80–85% RH and require the surface to be 5°F above the dew point.

Aliphatic vs aromatic, 1K vs 2K, and how polyurethane fits into coating systems for steel, concrete and roofs. With real data-sheet numbers, an ISO 12944 system builder and a DFT calculator, from a polyurethane manufacturer.

Coating System Help

Need the right polyurethane system? Ask the people who formulate it.

Tell us the substrate, the environment and how you'll apply it. Our technical team will recommend an ArmorThane polyurethane, polyurea or hybrid system with data sheets, or connect you with a certified applicator.

Liners, topcoats, membranes and spray foam
Steel, concrete, wood and fiberglass
Data sheets and application guidance
Contractors: ask about equipment and training

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Describe the Protective Coating Application You are Interested In.

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65–70%

Volume solids, typical 2K aliphatic topcoat

SW Hi-Solids, PPG 450H, Carbothane 134 HG

2–5 mils

Typical topcoat DFT on steel

Manufacturer data sheets

>25 yrs

ISO 12944 "Very High" durability

Not a guarantee period

0.02 ppm

OSHA ceiling for MDI

Why spray crews wear supplied air

"Polyurethane coating" means a clear finish on a tabletop to a homeowner, a glossy topcoat on a bridge girder to a corrosion engineer, and a sprayed truck liner to us. We've been formulating polyurethane and polyurea coatings since 1989, so this guide covers the industrial and protective side: what the chemistry actually is, aliphatic vs aromatic, how polyurethane fits into coating systems for steel and concrete, the numbers on real data sheets, and the safety rules that come with isocyanates.

Quick answer

A polyurethane coating forms when an isocyanate reacts with a polyol to create urethane links. It's tough, abrasion-resistant and flexible, and it comes as single-component (1K) moisture-cure products or two-component (2K) systems. Aliphatic polyurethanes hold color and gloss in sunlight, which makes them the standard topcoat over epoxy primers on steel, typically at 2–5 mils dry. Aromatic polyurethanes are cheaper and tougher but yellow in UV, so they're used as base coats, membranes and liners. On metal, polyurethane is almost always the last layer of a system, not the only one. Build a system for your environment.

What's in this guide

1How polyurethane coatings work
2Types of polyurethane coating and where they're used
3Aliphatic or aromatic? (selector)
4Polyurethane on metal: system builder
5DFT & coverage calculator
6Real data-sheet numbers
7Polyurethane waterproofing & floors
8Humidity, blisters and recoat windows
9Isocyanate safety and regulations
10Polyurethane vs polyurea vs epoxy
11Polyurethane coating FAQ

Chemistry

How polyurethane coatings work

Every polyurethane coating has two halves. The isocyanate (the "A side" in our world) carries reactive –NCO groups. The polyol (the "B side") carries –OH groups. When they meet, they form urethane linkages and build a cross-linked polymer. Change the polyol and you change the coating: polyester and acrylic polyols give hard, chemical-resistant topcoats; long polyether chains give soft, stretchy membranes.

Single-component, moisture-cure

A prepolymer with spare isocyanate groups that react with humidity in the air. No mixing, long open time, and they can go on damp or green concrete in some cases (Sikalastic-320 NS). Shelf life is limited, around six months, because any moisture that gets into the can starts the cure.

Two-component

Isocyanate and polyol mixed just before use, with a pot life of typically 2–6 hours. More controlled cure, higher performance, and the format of virtually every industrial topcoat and spray-applied liner. Full properties take days; one source puts full cure around 14 days at 25°C.

Aromatic vs aliphatic is the other big split. Aromatic isocyanates (MDI, TDI) contain benzene rings, react fast and cost less, but yellow and chalk in sunlight. Aliphatic isocyanates (HDI, IPDI, H12MDI) don't absorb UV the same way, so they keep color and gloss outdoors. That's why the same coating system often uses both: an aromatic base for toughness and an aliphatic top for appearance.

Polyurethane vs polyurea: if the B side is an amine instead of a polyol, the product is a polyurea, which reacts in seconds and is far less sensitive to humidity. Hybrids sit in between. We've written a full comparison: polyurea vs polyurethane, plus urethane vs polyurethane for the naming question.

Map of the family

Types of polyurethane coating and where they're used

Type	Format	Typical build	Used for	Example
Aliphatic acrylic or polyester PU topcoat	2K, solvent or high-solids	2–5 mils DFT	Steel structures, tanks, equipment, OEM finishes	SW Hi-Solids B65-300; PPG Amercoat 450H; ArmorThane UV TopCoat
Moisture-cure waterproofing membrane	1K	30–60 mils	Below-grade, planters, balconies	Sikalastic-320 NS (95% solids)
Traffic / deck coating system	Aromatic base + aliphatic top	≈35–40 mils	Parking decks, plazas, balconies (ASTM C957)	Sikalastic 720/745 AL; ArmorDeck
Roof coating over SPF	1K moisture-cure (ASTM D6947)	≈30–40 mils total	Spray foam roofs	Aromatic base + aliphatic top
Sprayed polyurethane elastomer liner	2K, low-pressure	Built to spec	Bedliners, trailers, equipment, containment	ArmorThane STS-200 / STS-300 (2:1)
Polyurethane floor / urethane cement	2K or 3-part	Coatings to ¼ in mortars	Food plants, hot washdowns, garages (topcoats)	Urethane mortar systems
Spray polyurethane foam	2K, plural component	Inches	Insulation, roofing, flotation	ArmorFoam closed- and open-cell

Builds from manufacturer data sheets and the US Made Supply polyurethane roof coating guide. The roof system build (20–25 mil base + 10–15 mil top) is a typical specification, not a standard requirement.

Tool 1 of 3

Aliphatic or aromatic? Quick selector

Isocyanate selector

Which polyurethane chemistry fits this layer?

A rule-of-thumb guide; the coating system specification governs.

Role of this layer

Sunlight reaches it?

Color or gloss matters?

Humidity during application

—

Tool 2 of 3

Polyurethane on metal: system builder

On steel, polyurethane is the weather-resistant skin of a multi-coat system. ISO 12944 classifies environments from C1 (heated interiors) to CX (extreme offshore and industrial) and durability from Low (under 7 years) to Very High (over 25 years). Pick both, and the builder shows a typical layer stack drawn from certified Sherwin-Williams ISO 12944 systems.

ISO 12944 system builder

Typical stack for your steel

Illustrative systems, not a specification. Verify with the coating manufacturer's certified systems.

EnvironmentFirst-year steel corrosion rate

Durability target

Exposed to sunlight?

—

Tool 3 of 3

DFT & coverage calculator

Theoretical coverage = 1,604 × volume solids ÷ DFT in mils (or 10 × volume solids ÷ DFT in microns, for m² per liter). Check: Sherwin-Williams Hi-Solids at 65% solids works out to about 1,043 sq ft per gallon at 1 mil, matching its published 1,040; Carbothane 134 HG at 70% and 50 µm gives 14 m²/L, matching its sheet. Then subtract application losses.

Coverage calculator

How much coating to order

Loss factors are rules of thumb; adjust for your crew, wind and surface.

Area (sq ft)

Volume solids (%)

Target DFT (mils)

Coats

Application method (loss)

Brush / roll (5–10%)Airless spray (20–30%)Plural component (≈15%)

—

Theoretical sq ft/gal

—

Practical sq ft/gal

—

Wet film to apply (mils)

—

Gallons to order

On the data sheet

Real data-sheet numbers for 2K aliphatic topcoats

Property	SW Hi-Solids Polyurethane (B65-300)	PPG Amercoat 450H	Carboline Carbothane 134 HG
Volume solids	65% ±2%	67% ±2%	70% ±2%
DFT per coat	3–5 mils	2–3 mils	50–75 µm (2–3 mils)
VOC	<300 g/L	311.5 g/L	264 g/L
Pot life	4 h	4 h	4 h
Recoat	18 h min, 30 days max (77°F)	4 h min, no max (70°F)	8 h (24°C); 36 h (2°C)
Max humidity	85% RH	85% RH	80% RH
Performance	Taber 87.1 mg; adhesion 1,050 psi; 9,000 h salt fog	Over zinc or epoxy primers	SSPC Paint 36 Level 3; dry heat to 149°C

Sources: manufacturer product data sheets linked below. SSPC Paint 36 is the performance standard for two-component weatherable aliphatic polyurethane topcoats.

For gloss retention, the gap between chemistries is large. In accelerated QUV-A testing summarized by PCI Magazine, acrylic urethanes took 3,800–6,600 hours to lose 25% of their gloss, while a urethane-crosslinked fluoropolymer went past 10,000 hours. Aromatic polyurethanes aren't in the same race; they're not meant to be the layer the sun sees.

Concrete & decks

Polyurethane waterproofing & floor coatings

Waterproofing membranes. Polyurethane's elongation is what makes it a waterproofer. Sika's Sikalastic 720/745 AL traffic system lists 800% elongation, 2,500 psi tensile and 300 pli tear, meets ASTM C957 for traffic-bearing membranes and carries a Class A fire rating (ASTM E108). The single-component Sikalastic-320 NS goes on at 25 sq ft per gallon for 60 mils, works from −25 to 200°F and tolerates damp concrete. That flexibility is why polyurethane membranes are specified on balconies, plaza decks and planters where concrete moves. ArmorThane's ArmorDeck system follows the same pattern: a moisture-cured base membrane with an aliphatic topcoat.

Floors. On garage and commercial floors, polyurethane usually appears as the clear topcoat over an epoxy base, adding abrasion and UV resistance. In food plants, urethane cement mortars ($8–$15 per sq ft installed vs $5–$8 for high-build epoxy, per AICoat) handle hot washdowns and thermal shock that crack epoxy. See our epoxy floor coating and garage floor coating guides.

Roofs. Over spray foam, moisture-cure polyurethane roof coatings are covered by ASTM D6947. A typical system is a 20–25 mil aromatic base with a 10–15 mil aliphatic top, about 30–40 mils total. More in our spray-on roof coating guide.

Troubleshooting

Humidity, blisters and recoat windows

Isocyanates react with water as well as polyol, and that side reaction releases carbon dioxide. Apply a 2K polyurethane in high humidity, or over a damp surface, and the CO2 gets trapped as fine bubbles or "foaming" that can weaken intercoat adhesion (KTA-Tator). That's why topcoat data sheets cap humidity at 80–85% RH and require the steel to be at least 5°F (3°C) above the dew point.

Check dew point, not just RH. Steel at dawn is often colder than the air.
Respect the recoat window. Sherwin-Williams Hi-Solids allows 18 hours to 30 days at 77°F; beyond the maximum, scuff-sand before recoating.
Don't over-thin. Extra solvent raises VOC and can push DFT below spec; measure wet film as you go.
Consider polyurea where humidity or cold is unavoidable. Amine-cured polyureas are far less moisture-sensitive and cure in seconds.

Non-negotiable

Isocyanate safety and regulations

Diisocyanates are respiratory sensitizers: once someone is sensitized, very small exposures can trigger asthma. The risk is highest while the product is uncured and especially when it's sprayed. Once fully cured, polyurethane coatings are inert plastics.

United States

OSHA's ceiling for MDI is 0.02 ppm (0.2 mg/m³), and OSHA runs a National Emphasis Program on isocyanates (CPL 03-00-017). EPA requires 90-day notice for consumer coatings and sealants with more than 0.1% TDI under a significant new use rule.

European Union

Since 24 August 2023, REACH Entry 74 requires training before professional use of products with 0.1% or more diisocyanates, renewed every five years. Spraying outdoors or with limited ventilation requires the advanced level.

On the job

Supplied-air or appropriate respirators for spraying, skin and eye protection, ventilation, setbacks for other trades, and safety data sheets on site. Every ArmorThane applicator course covers this before anyone sprays.

Compare

Polyurethane vs polyurea vs epoxy

	Polyurethane	Polyurea	Epoxy
Cure speed	Hours (2K) to days (1K)	Seconds to minutes	Hours; 7-day full cure
UV stability	Aliphatic: excellent	Aliphatic grades only	Ambers and chalks
Flexibility	Low to very high	High	Rigid
Moisture sensitivity during cure	High (CO2 bubbling)	Low	Moderate (blush)
Adhesion to concrete / steel	Usually over a primer	Usually over a primer	Excellent primer
Typical role	Topcoats, membranes, liners, foam	Liners, waterproofing, containment	Primers, floors, tank linings

More: polyurea vs other coatings and ArmorThane polyurethane products.

Our line

ArmorThane polyurethane coatings

Liner

STS-200 & STS-300

2:1 low-pressure polyurethane liners free of mercury, VOCs and TDI, for metal, concrete, fiberglass, wood and some plastics.

Hybrid

FlexGard 75

A high-pressure hybrid that can be milled to exact tolerances after cure.

Topcoat

UV TopCoat

Two-component acrylic polyurethane topcoat, semi-gloss, with 3.5 lb/gal VOC and strong chemical and exterior resistance.

Topcoat

UV ColorCoat

Aliphatic polyurethane topcoat for UV protection and custom color matching over liners.

Contractors & OEMs

Need a polyurethane system? Talk to the people who formulate it.

Tell us the substrate, the environment and how it's applied. Our technical team will recommend an ArmorThane polyurethane, polyurea or hybrid system with data sheets, and connect you with an applicator or set you up to apply it yourself.

Ask our technical team Browse products

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Straight answers

Polyurethane coating FAQ

What is polyurethane coating used for?

Weather-resistant topcoats on steel structures and equipment, waterproofing membranes on decks and below-grade concrete, roof coatings over spray foam, sprayed liners for trucks and equipment, floor topcoats, and spray foam insulation. Clear polyurethanes are also common wood finishes.

Is polyurethane coating waterproof?

Yes. Elastomeric polyurethane membranes are used specifically for waterproofing, and products like Sikalastic-320 NS meet ASTM C836 for below-grade waterproofing. Thin decorative polyurethane topcoats are water-resistant but aren't designed as membranes.

Can you put polyurethane coating on metal?

Yes, as part of a system. Steel is blasted (typically SSPC-SP6 minimum, SP10 for severe service), primed with zinc or epoxy, then topcoated with an aliphatic polyurethane at about 2–5 mils. In ISO 12944 C4–C5 environments, systems of roughly 240–300 µm total are typical.

What's the difference between aliphatic and aromatic polyurethane?

Aromatic polyurethanes (based on MDI or TDI) are tougher per dollar but yellow and chalk in sunlight, so they're used as base coats, membranes and liners. Aliphatic polyurethanes (HDI, IPDI, H12MDI) hold color and gloss outdoors and are used as topcoats.

What is a 2K polyurethane coating?

A two-component coating: an isocyanate hardener mixed with a polyol resin just before application. It cures by chemical reaction, typically has a pot life of 2–6 hours and outperforms single-component (1K) moisture-cure products in most industrial uses.

How long does polyurethane coating last?

As part of a protective system on steel, ISO 12944 durability ranges run from under 7 years (Low) to over 25 years (Very High), depending on the environment and system. Topcoats are often refreshed during that life to restore gloss and color.

How thick should a polyurethane coating be?

Topcoats on steel are typically 2–5 mils dry. Waterproofing membranes are 30–60 mils, and traffic deck systems around 35–40 mils. Always follow the product's data sheet.

Is polyurethane coating toxic?

Uncured isocyanates are respiratory sensitizers, so spraying requires proper respiratory protection, ventilation and training. OSHA's ceiling for MDI is 0.02 ppm. Fully cured polyurethane is an inert plastic.

Is polyurethane better than epoxy?

For different jobs. Epoxy is the better primer and adheres strongly to concrete and steel; aliphatic polyurethane is the better topcoat for UV, gloss retention and abrasion. Many systems use both: epoxy underneath, polyurethane on top.

Why is my polyurethane coating bubbling?

Usually moisture. Isocyanates react with water to form CO2, which gets trapped as bubbles when applied in high humidity, over damp surfaces or too thick. Data sheets typically cap humidity at 80–85% RH and require the surface to be 5°F above the dew point.

Written by the ArmorThane technical team. We've formulated and manufactured polyurethane and polyurea coatings since 1989 and train applicators across North America. Updated October 2026.

Sources

Sherwin-Williams Hi-Solids Polyurethane B65-300 product data sheet: paintdocs.com
PPG Amercoat 450H product data sheet: ppg.com
Carboline Carbothane 134 HG product data sheet (distributor copy): altexcoatings.com
Sikalastic 720/745 AL traffic system: usa.sika.com (PDF)
Sikalastic-320 NS: usa.sika.com
SSPC Paint 36 summary: rustbullet.com.au
PCI Magazine, 2-component polyurethane topcoats: pcimag.com
Sherwin-Williams ISO 12944:2018 systems guide: sherwin-williams.com (PDF)
ISO 12944 corrosivity categories: worldofcorrosion.com
Cloverdale Paint, ISO 12944 systems guide: cloverdalepaint.com (PDF)
SSPC abrasive blast cleaning standards: freerconsulting.com
US Made Supply, polyurethane roof coating guide: usmadesupply.com
SpecialChem, polyurethane coatings guide: specialchem.com
KTA-Tator, blistering and bubbling of coatings: kta.com
ScienceDirect, polyurethane coating topic overview: sciencedirect.com
OSHA, MDI chemical data: osha.gov/chemicaldata/23
OSHA, isocyanates National Emphasis Program: osha.gov
Finnish OSH Administration, REACH diisocyanate training requirements: tyosuojelu.fi
EPA, toluene diisocyanate fact sheet: epa.gov
AICoat, commercial epoxy flooring cost: aicoat.com

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Polyurethane Coating: Types, Specs & Systems

Need the right polyurethane system? Ask the people who formulate it.

Where are you Located?

How polyurethane coatings work

Single-component, moisture-cure

Two-component

Types of polyurethane coating and where they're used

Aliphatic or aromatic? Quick selector

Which polyurethane chemistry fits this layer?

Polyurethane on metal: system builder

Typical stack for your steel

DFT & coverage calculator

How much coating to order

Real data-sheet numbers for 2K aliphatic topcoats

Polyurethane waterproofing & floor coatings

Humidity, blisters and recoat windows

Isocyanate safety and regulations

United States

European Union

On the job

Polyurethane vs polyurea vs epoxy

ArmorThane polyurethane coatings

STS-200 & STS-300

Need a polyurethane system? Talk to the people who formulate it.

Polyurethane coating FAQ

Sources

latest news & bLOGS

Polyurethane and polyurea, made by us since 1989.

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Polyurethane
Coating: Types,
Specs & Systems