Bostik Simson ISR 70-08 vs 70-03: What’s the Difference and Which Should You Use?

What’s the Difference Between ISR 70-03 and ISR 70-08?

When choosing a professional-grade sealant or adhesive, small numbers can hide big differences. At first glance, Bostik Simson ISR 70-08 and ISR 70-03 look like close cousins. Same family, same trusted brand, same reputation across marine, automotive, transport and industrial sectors. Need a waterproof seal? Read our guide Best Adhesive for Waterproof Sealing (UK Guide)

But in practice, they behave very differently once they leave the cartridge.

This guide breaks down the real-world differences between the two, explains where each one shines, and helps you confidently choose the right option for your application. If your application involves bonding different materials, see our strongest glue for metal to plastic guide for high-strength options.

If you’re comparing bonding systems more broadly, you may also want to read our complete Structural Adhesives UK (Trade Edition) guide, which explains how different adhesive chemistries perform in load-bearing and sealing applications.

---

An Overview of the Bostik Simson ISR Range

The Bostik Simson ISR range is built around silane-modified polymer (SMP) technology.

This gives you a rare blend of:

• Strong adhesion

• Long-term elasticity

• Excellent resistance to weathering

• UV and water resistance

Unlike traditional polyurethanes, SMP sealants are typically isocyanate-free, making them easier and safer to work with while still delivering industrial performance. For a broader comparison of adhesive types, read our hybrid polymer vs PU vs silicone guide.

If you’re weighing SMP sealants against structural bonding systems like methacrylate’s, our guide to when to bond instead of bolt provides useful context.

Both ISR 70-08 and ISR 70-03 are available in:

• Colours: White & Black

• Sizes: 290ml cartridges and 600ml sausages

Where they differ is in stiffness, movement capability and intended use.

---

What Is Bostik Simson ISR 70-03?

ISR 70-03 is the more flexible option of the two.

Think of it as a sealant that likes to move with the materials it bonds. It is designed for joints and assemblies where vibration, expansion or regular movement are part of daily life. For sealing and flexible bonding applications, our best adhesive for waterproof sealing guide compares the most reliable systems.

Typical uses include:

• Vehicle body sealing

• Marine deck fittings

• HVAC and panel joints

• General elastic sealing applications

If your joint behaves like a hinge rather than a brick wall, ISR 70-03 usually feels right at home.

For applications involving flexible rubber interfaces or expansion joints, you may also find our rubber bonding adhesive comparison guide useful.

---

What Is Bostik Simson ISR 70-08?

ISR 70-08 is noticeably stiffer.

It still offers elasticity, but its higher modulus makes it better suited to structural bonding and load-bearing applications where strength and shape retention matter more than extreme movement. If you require higher strength bonding, see our structural adhesives UK guide for MMA and epoxy alternatives.

Common applications include:

• Bonding panels and components

• Transport and trailer construction

• Industrial assembly

• Situations where a firmer adhesive line is required

If ISR 70-03 is a flexible joint specialist, ISR 70-08 is the disciplined structural performer.

For rigid metal bonding applications, our best adhesive for metal to metal guide compares structural options.

---

Side-by-Side Comparison Chart

---

Advantages at a Glance

ISR 70-03 Advantages

• Excellent elasticity for moving joints

• Strong adhesion to a wide range of substrates

• Ideal for vibration-prone environments

• Clean, professional finish in white or black

ISR 70-08 Advantages

• Higher strength and rigidity

• Better shape retention under load

• Ideal for bonding panels and components

• Reliable performance in demanding industrial settings

Both products cure by reacting with moisture in the air and form a durable, rubber-like bond that resists weathering and ageing. If your application involves bonding rubber or elastomeric components to metal, you may also want to review our detailed Bostik 2402 rubber bonding guide, which covers two-part contact systems. If bonding rubber to metal or flexible materials, see our best adhesive for rubber bonding guide.

---

Which One Should You Use?

Real-World Applications

• ISR 70-03 used for sealing vibrating vehicle panels or marine joints
• ISR 70-08 used for bonding aluminium panels in transport builds
• ISR 70-03 for HVAC duct sealing where movement occurs
• ISR 70-08 for fixing structural trims or components

👉 Choosing the right modulus ensures long-term performance and avoids premature failure.

Choosing between these two is less about which is “better” and more about which one matches the job in front of you.

Choose ISR 70-03 if:

• The joint will expand, contract, or vibrate

• You are primarily sealing rather than bonding

• Flexibility is more important than stiffness

Choose ISR 70-08 if:

• You need higher mechanical strength

• The bond must support or stabilise components

• You want a firmer adhesive line with less movement

When professionals talk about the versatility of Bostik Simson ISR, this ability to choose stiffness levels without changing product families is a big reason why. If you are unsure whether your project calls for a sealant, structural adhesive or a high-strength MMA system, our Structural Adhesive Selection Guide provides a broader comparison.

When NOT to Use ISR Sealants

Avoid ISR 70-03 and 70-08 when:

• Bonding low surface energy plastics (PP or PE)
• You need true structural load-bearing strength
• The application involves continuous water immersion

👉 In these cases, a structural adhesive such as an MMA or epoxy system may be more suitable. For low surface energy such as PP or PE you can use Permabond TA4611 designed to bond plastic without a primer.

---

Compliance and Standards

Products in the Simson ISR range are commonly used in applications aligned with recognised construction and industrial standards. For a deeper look at how elastic sealants are classified and tested, you can refer to the international sealant standard ISO 11600, which outlines performance requirements for building sealants. This provides useful context for understanding modulus and movement capability in real applications. ISO 11600 sealant standard (ISO) — This official entry from the International Organization for Standardization outlines how sealants are classified and tested based on performance and joint movement capability, helping professionals understand the right sealant choice for different applications.

---

Where to Buy

For guaranteed genuine stock and trade-friendly options, you can purchase both products here:

• 👉 Buy Bostik Simson ISR 70-03 from Adhesive Supply Company

• 👉 Buy Bostik Simson ISR 70-08 from Adhesive Supply Company

Both are available in white or black, and in 290ml cartridges or 600ml sausages, so you can match the format to your application and tools.

---

Final Thoughts

The difference between ISR 70-03 and ISR 70-08 is not subtle once you understand how they are designed to behave. One flexes gracefully with movement, the other stands firm and carries load. By matching the sealant’s personality to your application, you get cleaner results, longer service life, and fewer call backs.

If you remember just one thing, remember this: choosing the right Bostik Simson ISR product is less about numbers on a datasheet and more about how your joint lives and breathes in the real world. For more technical advice, explore our adhesive advice & technical guides covering bonding, sealing and material selection.