Expanding Foam vs. Acrylic Sealant: Which Is Best for Sealing Gaps in Interior Walls?

Sealing gaps in interior walls is essential for both a high-quality finish and thermal efficiency on any Australian build or renovation. But when you’re facing a variety of gaps, from large voids to hairline cracks, choosing the wrong product creates headaches like shrinkage, cracking, or bulging.

This guide compares the two most popular strategies, expanding foam and acrylic sealant, to determine the best gap filler for walls based on size and function. We’ll show you how to use Soudal Gap Filling Expanding Foam and Genius Gun for large voids, Light Speed Filler for the perfect finish, and the essential role of Universal BOG for repairs.

Key Takeaways

• For large voids (greater than 10 mm), use Pro Expanding Foam. For small finishing joints (less than 10 mm), use Pro Gaps Light Speed acrylic sealant.
• Acrylic sealant offers moderate elasticity and is paintable soon after application. Pro Expanding Foam provides great thermal insulation (-40°C to +90°C) but must be over-coated.
• To prevent shrinkage, apply acrylic sealant in layers when filling deep gaps. Always over-coat cured foam to protect the non-UV resistant material and achieve a flawless surface.

Expanding Foam: Gap Fillers for Voids

Expanding foam is a robust, polyurethane-based solution designed for void filling and thermal/acoustic insulation. It’s not a conventional elastic sealant but rather a lightweight foam. Products like Soudal Gap Filling Expanding Foam (available in straw-applied or Genius Gun formats) expands up to 30 times its original volume to fill large, irregular spaces.

Use Cases & Benefits of Expanding Foam

• Filling Large Gaps: Ideal for filling large gaps (10mm+), insulating around window and door frames, as well as sealing service penetrations in walls.
• Insulation & Stability: Provides excellent thermal and acoustic insulation and creates an airtight seal that contributes to energy efficiency.
• Specialised Options: For joints with movement, products like Flexifoam are formulated for greater elasticity than standard PU foams, allowing them to absorb substrate movements without cracking.

Mistakes to Avoid

The most frequent error is overfilling large gaps, leading to outward pressure and a poor finish. The second mistake is not coating foam. Leaving cured foam exposed compromises the finish, as the foam is not UV resistant. It must be cut back, sanded, and over-coated with a flexible sealant or filler for a clean, paintable surface. 

Acrylic Sealant & Bog Fillers: Gap Fillers for Finishing

When the goal is a flawless, paintable surface, you need a different solution. Next, we explore flexible sealants for small movement and rigid fillers for static repairs.

Acrylic Sealant for Finishing

Acrylic sealant is a water-based joint sealant designed for seams that require a flexible, paint-ready finish. Soudal Fill & Paint is a high-quality acrylic sealant designed for very easy application and excellent finish quality.

• Uses: Perfect for small interior joints (with movement up to 10%) and finishing seams, such as sealing the joint between plinths and walls (skirting boards) or window sills. It’s also suitable for filling cracks in concrete and plasterwork.
• Benefits: Very easy to apply, colourfast and waterproof after curing, and provides very good adhesion on porous substrates like plasterwork and even aluminium.

Universal BOG for Repairs

For patching surface defects and rigid repairs where sanding is necessary, Universal BOG (a two-component polyester putty) is the solution. It offers a hard, durable fill with minimal shrinkage, making it ideal for plasterboard damage or minor concrete defects before sanding and painting.

Mistakes to Avoid

When using acrylic sealant for gap filling, do not try to fill large holes in one go. This causes shrinkage and cracking as the water base dries out. Apply the acrylic in layered applications for large gaps. Another common mistake to avoid is using temporary materials (like scraps of cardboard or paper) as backing before filling, as this leads to unstable, non-compliant seals.

Expanding Foam vs. Acrylic Sealant: Key Differences in Application

To make sure you choose the best gap filler every time, here’s a clear breakdown, comparing expanding foam, acrylic sealant, and Universal BOG Filler side-by-side.

Best Practices: When to Choose Each Gap Filler

When it comes to the final finish, knowing which product to grab prevents rework. Here are the gap fillers your team recommends, based on your next job.

• Use Gap Filling Expanding Foam For: Major gaps and voids needing insulation or draft-proofing, like pipe penetrations or large wall cavities. Importantly, it must be over-coated with a sealant or filler for a durable finish.
• Use Fill & Paint For: All visible finishing joints and seams where flexibility and paintability are required, such as where skirting meets the wall. This provides a smooth, professional finish that blends perfectly with paint.
• Use Universal BOG For: Rigid surface repairs where the filler must be sanded and painted smooth, such as patching major plasterboard damage or defects. It provides a hard, non-flexible repair.

Conclusion: Mastering Gap Fillers for a Flawless Finish

The choice between expanding foam vs acrylic sealant boils down to function. Use foam for structure and insulation in large voids, acrylic for a flexible, clean, paintable finish in small gaps, and bog filler for rigid surface repairs.

By correctly matching the product to the task, you ensure the highest quality finish and efficiency, guaranteeing a long-lasting result from your wall gap filler. 

To achieve a flawless, professional result every time, explore the full Soudal range of expanding foams and sealants and find your nearest store today.