Bond Repair: Making Surface Prep Measurable
Unlike pristine manufacturing environments where parts are prepped and assembled under controlled conditions, repairs often happen in less-than-ideal circumstances: an aircraft hangar, a wide-open warehouse, or even in the field.
Contamination risks skyrocket in uncontrolled environments. Jet fuel residue, oil, grease, dust, and humidity are all potential threats to a successful bond. On top of that, repair technicians are often working manually, sanding and/or cleaning the surface by hand. Every stroke of sandpaper increases the odds of over-sanding, potentially exposing reinforcing fibers and reducing the lifetime of the repair.
Why Repair Bonding Is So Difficult
The Composite Materials Handbook (CMH-17), the global standard for composite design and repair, emphasizes that:
“Surface preparation is often the most critical factor in achieving a reliable bonded joint. Improper or inconsistent surface preparation can lead to reduced bond strength and premature failure.” — CMH-17, Vol. 3
The challenge comes down to three realities of repair work:
• Manual variability – No two technicians sand or prep exactly the same way.
• Uncontrolled environments – Field and hangar repairs have numerous contaminants.
• Risk of oversanding – Exposing reinforcing fibers by abrasion can make bonding worse, not better.
Unlike manufacturing processes that rely on automation and repeatability, repair work often depends on technician judgment and visual inspection. Unfortunately, many critical surface conditions cannot be seen with the naked eye.
Surface Roughening Isn’t Always Beneficial
Surface roughening is commonly used to improve mechanical adhesion, but in composite repair it is rarely a perfectly controlled or uniform process. In aerospace and industrial repair procedures, technicians are often instructed to “sand to black” when preparing carbon fiber-reinforced composites for bonding.
While widely used, this approach leads to exposure and damage to reinforcing fibers: the ‘black’ in sand to black is debris from fractured and abraded carbon fibers. This can negatively impact bond performance and long-term repair durability.
Sanding can also introduce contamination risks of its own. Many abrasive materials contain low-energy additives, such as stearates, which are designed to extend abrasive life, but may transfer onto the composite surface during preparation. These residues can reduce surface energy and interfere with adhesion.
When combined with already challenging repair environments, uncontrolled sanding introduces another layer of variability that makes consistent bonding difficult to achieve.
Fiber Exposure: The Hidden Enemy
One of the most common repair pitfalls is sanding too aggressively. While it might look “clean,” oversanding can expose fibers, which reduces bond performance.
The CMH-17 warns: “Excessive abrasion can damage fibers, introduce defects, and reduce adhesion. Optimal preparation achieves a clean surface without compromising the composite substrate.” — CMH-17, Vol. 3
Figure 1: Over sanding increases water contact angle
Measurements show that fiber exposure from over sanding increased contact angle, reducing wettability and making surfaces less favorable for bonding (orange). Meanwhile, atmospheric plasma treatment lowered water contact angle significantly (green). Our Surface Analysts™ make this visible, helping technicians avoid over abrasion.
Figure 2. ATR-FTIR of over-abraded epoxy composite
We confirmed through ATR-FTIR analysis that over sanding resulted in total removal of the epoxy resin, as evidenced by the disappearance of the aliphatic hydrocarbon and aromatic ether peaks– exposing the carbon fiber and likely reducing bondability.
Building Confidence in Repair Work
By validating surface prep with objective, real-time data, technicians gain confidence that their repairs are done right — even in tough conditions. The benefits include:
• Reliable confirmation before bonding
• Consistency across different operators
• Reduced risk of oversanding or contamination issues
• Alignment with industry standards like CMH-17
Bond repairs will always be challenging to control. But by making surface prep measurable, Brighton Science enables aerospace teams to ensure that every repair bond is strong, reliable, and built to last.
Learn More About Validating Composite Surface Prep
Ensuring every composite repair bond is reliable starts with understanding and validating surface preparation. For a deeper dive into best practices, including qualification of surface processes for bonded aircraft repair, check out our technical paper: Qualification of Surface Processes for Bonded Aircraft Repair.
Brighton Science provides the tools and data you need to make surface prep measurable — giving repair teams confidence that every bond will perform as intended.
