Choosing the right sealant can make or break a build. Whether you are filling architrave gaps before a topcoat or caulking expansion joints in a concrete driveway, grabbing the correct tube guarantees a job that lasts under Australia’s harsh conditions. Two of the most common products on any job site are polyurethane (PU) and acrylic. While both fill gaps and block draughts, they handle weather, movement, and moisture very differently.
What Is Polyurethane Sealant?
Polyurethane sealant is a heavy-duty flexible rubber sealant built for high-movement joints, outdoor weatherproofing, and tough structural bonds. It cures by reacting with air moisture, creating a strong, elastic seal that sticks like glue to concrete, brick, steel, timber, and fiber cement. Because polyurethane stays rubbery and tough under heavy stress, it won’t crack or tear when materials expand in the heat or shrink in the cold. It handles harsh Aussie sun, torrential rain, and structural movement without flinching.
Key Features of Polyurethane:
- High Movement Allowance: Stretches and compresses up to ±25% (or more) without snapping or pulling away from edges.
- Weather & UV Proof: Holds up against direct sunlight, driving rain, and constant moisture over many years.
- Paintable: Takes both water-based and oil-based paints once fully cured.
- Binds Heavy Materials: Sticks tight to porous and non-porous surfaces like brickwork, concrete slabs, metal flashing, and timber posts.
What Is Acrylic Sealant?
Acrylic sealant, commonly called gap filler or painter’s caulk, is a water-based filler designed for fast indoor gap sealing and pre-paint prep work. It dries as water evaporates from the bead, leaving a smooth surface that takes paint clean and easy.Acrylic gap fillers are budget-friendly, quick to gun out, and clean up in seconds with water. While great for internal trim, they lack the extreme stretch and water resistance needed for structural outdoor work.
Key Features of Acrylic:
- Easy Water Clean-up: Uncured filler wipes away cleanly with a wet rag, no solvents or turps required.
- Fast Paint Turnaround: Skins over fast and takes water-based acrylic paint cleanly without cracking the topcoat.
- Low Cost: An economical choice when prepping hundreds of meters of interior trim and plasterboard joints.
- Low-Movement Fit: Perfect for stable internal gaps around skirting boards, architraves, and ceiling cornices.
Polyurethane vs. Acrylic Sealants
The main difference between polyurethane and acrylic sealants comes down to stretch, weatherproofing, and clean-up. Polyurethane is your heavy-duty exterior joint sealer, while acrylic is your quick indoor gap filler before the paintbrush comes out.
| Property | Polyurethane Sealant (PU) | Acrylic Sealant (Gap Filler) |
| Best Used For | Exterior expansion joints & heavy construction | Internal prep work & gap filling before painting |
| Movement & Stretch | High flexibility (±25% to ±50% movement) | Low flexibility (±7.5% to ±12.5% movement) |
| Water Resistance | 100% waterproof; handles wet weather | Water-resistant when dry; washes out if wet before curing |
| Cure Time | Slower (24–48 hours for full cure) | Fast (skins in 20–30 mins, paintable quickly) |
| Clean-Up | Mineral spirits / turps / solvents | Plain warm water and a wet rag |
| Paint Ability | Paintable once fully cured | Excellent paintability; designed for painter’s prep |
| Cost | Premium trade price | Budget-friendly / bulk filler |
When Should You Use Polyurethane Sealant?
You should use polyurethane sealant for outdoor expansion joints, weatherproofing external penetrations, sealing cladding, and high-vibration structural gaps. Whenever a joint will take a beating from rain, sun, or structural shifting, reach for polyurethane.
Top Jobs for Polyurethane:
- Sealing expansion and control joints in concrete driveways, pathways, and retaining walls.
- Filling vertical control joints between fiber cement sheets, brickwork, and stone panels.
- Caulking perimeter frames against brickwork or weatherboard exteriors to stop water leaks.
- Sealing metal roof sheets, ridge capping, gutters, and downpipe joins.
When Should You Use Acrylic Sealant?
You should use acrylic sealant for internal gap filling around timber architraves, skirting boards, cornices, and plasterboard wall trim. When you need a smooth, gap-free paint finish indoors with minimal mess, acrylic is the go-to product.
Top Jobs for Acrylic:
- Sealing narrow gaps between timber trim and plasterboard walls prior to painting.
- Tidying up corner joints along Gyprock or Siniat plaster cornices.
- Filling small gaps around interior timber jambs and window architraves.
- Filling low-movement hairline cracks in interior walls before applying primer.
Recommended Products
Building Supplies Australia Network stocks trade-proven sealants ready for your next site delivery:
FulaFlex 570FC PU
A high-performance, fast-curing polyurethane construction adhesive and sealant ideal for heavy expansion joints, cladding, and structural bonding.
Sikaflex 11FC
The industry-standard multi-purpose elastomeric polyurethane sealant and adhesive for concrete slabs, masonry, and exterior weatherproofing.
Soudal Seal & Stretch
A highly flexible, trade-quality acrylic sealant designed to accommodate movement in internal gaps, skirting boards, and architraves without cracking paint.
Bostik Gap Seal
A versatile, easy-to-use acrylic gap filler perfect for smooth pre-paint preparation around internal trim, windows, doors, and plasterboard joints.
Get Your Sealants & Building Supplies Delivered Fast
Getting the right sealant on-site saves costly callbacks down the track. Whether you need bulk acrylic gap filler for an interior fit-out or trade-grade polyurethane for structural concrete joints, we have you covered. Building Supplies Australia Network delivers direct to site across major Australian capital cities including Adelaide, Brisbane, Melbourne, and Sydney, as well as regional commercial hubs. Order online today or request a fast quote for your next build!