What Is A Polyurethane Sealant? Where And How To Use?

Key Takeaways on Polyurethane Sealants

  • Polyurethane sealants are extremely versatile and used for caulking cracks and joints. They come in one-part and two-part varieties.
  • They can bond to multiple surfaces like wood, metal, concrete, glass etc. Making them useful for both indoor and outdoor applications.
  • Polyurethane has better elasticity than silicone allowing it to expand and contract with surfaces. This makes it well-suited for moving joints.
  • While more expensive than other sealants, polyurethane offers better durability and longer service life. So it can be cost-effective in the long run.
  • Use polyurethane sealants anywhere regular caulking is needed. Common applications include windows, doors, siding, roofs, gutters, plumbing, HVAC etc.

What is a Polyurethane Sealant?

A polyurethane sealant is an elastic sealant used to fill gaps and cracks for waterproofing and insulation purposes. It cures to a rubber-like texture after application.

Polyurethane refers to a broad category of polymers derived from reacting polyols and diisocyanates. By tweaking the chemistry, polyurethanes can be manufactured as rigid plastics or flexible foams and sealants.

Sealants work by sticking to the surfaces of a joint while remaining elastic to handle expansion, contraction and mechanical vibrations. Polyurethane has properties that make it well-suited as a durable sealant.

Key Properties of Polyurethane Sealants

  • Adhesion – Bonds strongly to most building materials.
  • Flexibility – Has high elasticity and ability to stretch and compress with joint movement.
  • Durability – Resistant to weather, moisture, chemicals and wear. Long service life.
  • Versatility – Suitable for indoor and outdoor use. Bonds surfaces like wood, concrete, metal and glass.

These characteristics allow polyurethane caulk to effectively seal cracks and gaps exposed to weather or mechanical stress.

Types of Polyurethane Sealants

Polyurethane sealants come in one-part and two-part varieties:

  • One-Part – Cures by absorbing moisture from the air. No mixing required.
  • Two-Part – Cures via a chemical reaction when parts A and B are mixed. Produces a stronger, more rigid sealant.

Moisture-cured one-part polyurethanes are more commonly used. They cure evenly from surface to interior and come in tubes for use with caulking guns. Two-part versions require exact proportioning of resin and hardener.

Where To Use Polyurethane Sealants

Polyurethane caulks and sealants have hundreds of applications in construction, industry and maintenance:

Construction & Remodeling

  • Windows & doors – Seal frames to prevent air and water infiltration
  • Siding & roofs – Seal gaps around fixtures, vents, joints
  • Concrete & masonry – Expansion joints, control joints and cracks
  • Plumbing fixtures – Seal sinks, tubs and showers


  • Metal buildings – Seal metal panels, flashings and fastener heads
  • Transportation – Seal truck/trailer bodies
  • HVAC & ventilation – Seal ductwork and fan housings

Maintenance & Repair

  • Wood repair – Fill cracks and rotted sections before filling
  • Roof repair – Seal leaks, gaps and holes
  • Window glazing – Seal glass into frames
  • Plumbing leaks – Seal pipe joints and fittings
  • Docks, boats, ponds – Seal wood near water

Polyurethane works well as general gap-filling construction adhesive also. It bonds to most common building materials.

Advantages Over Other Sealants

Polyurethane sealants offer better performance than cheaper alternatives:

vs Silicone:

  • Bonds to more surfaces like wood and concrete
  • Higher elasticity – Handles +100%-200% joint movement
  • More durable in outdoor environments
  • Easier tooling into cracks and gaps

vs Acrylic & Latex:

  • Longer lasting – Decades rather than years
  • Bonds tightly to more surfaces
  • More flexibility – Expands and contracts better with joints
  • More weatherproof & UV/chemical resistant

vs Butyl Rubber:

  • Easier application – Applies smoothly from tubes
  • Cleaner look – Less residue or drooping
  • More versatile – Works on more indoor and outdoor surfaces

Over time, cheaper caulks like silicone and latex tend to crack, peel and fail – requiring repeated reapplication. Polyurethane’s durability saves rework and maintenance costs.

Though polyurethane costs 2-3X more upfront, it can be more cost effective long term while offering better performance.

How To Use Polyurethane Sealants

Using polyurethane sealants involve a similar process as other caulking:

1. Prepare the Joint

  • Clean all surfaces so they are free of old caulk, dust and oils
  • Widen and undercut any deep joints
  • Insert backer rod for deep joints to control sealant depth
  • Prepare good adhesion using primer if needed

2. Cut Nozzle & Apply Sealant

  • Cut the sealant nozzle at 45° for optimal bead shape
  • Gun sealant into joint smoothly, pushing sealant ahead of nozzle
  • Tool for even spread and contact with joint surfaces
  • Avoid air bubbles and gaps – fill the joint fully

3. Allow Proper Cure Time

  • Most skin over within 30-60 minutes
  • Cure rates depend on temperature and humidity
  • May take 24 hours to multiple days to fully cure
  • Avoid stressing joints before complete cure

Follow manufacturer guidelines on joint sizing, surface prep and application for best results.

Polyurethane Sealant FAQs

Q: Is polyurethane sealant waterproof?

A: Yes, fully-cured polyurethane has excellent waterproofing ability, blocking liquid water and moisture vapor.

Q: Does polyurethane stick to silicone?

A: In most cases, no. Polyurethane adheres poorly to residual silicone left from old caulk. Silicone areas must be thoroughly removed.

Q: Is polyurethane sealant UV resistant?

A: Polyurethane sealants have good UV resistance and typically don’t degrade from sun exposure. Its durability makes it suitable for outdoors.

Q: Can you paint over polyurethane sealant?

A: After full cure, polyurethane can be coated with latex paint. Oil-based paint tends to soften polyurethane. Always test compatibility.

Q: Is polyurethane toxic?

A: Uncured sealant has toxic isocyanate compounds, demanding safety precautions. But cured sealant is inert and non-hazardous.

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