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 build or reno. 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 Pro Expanding Foam for large structural voids, Pro Gaps Light Speed for the perfect finish, and the essential role of Gorilla BOG Filler for repairs.

Key Takeaways

• For large structural 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 resistance (-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: Heavy-Duty Gap Filler for Voids

Expanding foam is a robust, polyurethane-based solution designed for structural void filling and thermal/acoustic insulation. It’s not a conventional elastic sealant but rather a stiff, lightweight filler. Products in the Soudal Soudafoam range, like Pro Expanding Foam, expand significantly to fully fill large, irregular spaces.

Use Cases & Benefits

• Filling Large Gaps: Ideal for filling large gaps (10mm+), insulating around window and door frames, as well as sealing penetrations.
• Insulation & Stability: Provides excellent air and soundproofing and creates an airtight seal that contributes to energy efficiency.
• NZ Compliance: Many formulations, including Pro Expanding Foam, are BRANZ appraised for use as air seals around window and door penetrations, critical for meeting NZ building standards.

Mistakes to Avoid

The most frequent error is overfilling large gaps, leading to outward pressure and poor finish. The second mistake is not coating foam. Leaving cured foam exposed compromises the finish. Pro Expanding Foam 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 up, we explore flexible sealants for small movement and rigid fillers for static repairs.

Acrylic Sealant for Finishing

Acrylic sealant is a water-based, plasto-elastic material designed for joints and seams that require a flexible, paint-ready finish. Pro Gaps Light Speed is a high-performance acrylic sealant designed for fast-curing results.

• Uses: Perfect for small interior gaps (less than 10mm), hairline cracks in drywall, and finishing seams, like sealing the joint between skirting boards and walls.
• Benefits: Easy to use, paintable, and provides good adhesion to porous substrates like plasterboard.

Gorilla BOG Filler for Repairs

For patching surface defects and rigid repairs where sanding is necessary, Gorilla BOG Filler (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 severe 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 crumpled paper) as backing before filling with sealant, 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 Gorilla 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 our team recommends, based on your next job.

• Use Pro Expanding Foam For: Major structural 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 Pro Gaps Light Speed 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 Gorilla BOG Filler 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 stockist today.

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