This video is an amateur but thorough investigation into the optimal surface preparation for creating strong and, more importantly, consistent epoxy bonds on fiberglass composites for high-performance rocketry. The creator conducts nearly 100 break tests on cantilevered coupons, evaluating various methods including no prep, solvent wipes (acetone, IPA), a range of sandpaper grits (40 to 1000), and unconventional techniques like plasma and flame treatment.
While low-grit sandpaper (60 grit) initially produced the highest absolute bond strength (~4000 N), the results proved highly inconsistent and difficult to replicate. Subsequent tests were plagued by wide deviations, likely due to uncontrolled variables like sanding technique and potential contamination. The key finding, derived from an encounter with a launch vehicle engineer, is that consistency trumps peak strength. A final, conclusive test series demonstrated that a simple two-step solvent wipe—first with acetone, then with isopropyl alcohol (IPA)—yields highly consistent and reliable bond strengths around 3200 N. As far as I can tell from this data, adding sanding to this process offers no significant benefit. The creator correctly emphasizes that this is garage science, not peer-reviewed data, but the process highlights the value of prioritizing process control and consistency in engineering.
A YouTuber's Quest for the Best Composite Bond
0:00 The Problem: Inconsistent Advice: The video sets out to experimentally determine the best surface preparation for epoxy bonding on composites, as common advice is contradictory and unreliable.
1:20 Testing Methodology & Disclaimers: The creator uses a hydraulic press and a load cell to break cantilevered fiberglass coupons bonded with epoxy. He repeatedly and correctly warns that his tests are not lab-grade, have small sample sizes, and are anecdotal evidence at best.
Wipes with Isopropyl Alcohol (IPA) provided consistent but mediocre results (~3300 N).
Wipes with acetone alone performed poorly, possibly damaging the resin.
Sanding showed a clear trend: lower grit performed better. 60 grit sandpaper achieved the highest peak strength of the entire investigation (~4000 N), while 1000 grit was the worst.
10:45 The Struggle to Replicate Peak Strength: Subsequent attempts to replicate the 4000 N result using different 60-grit sanding techniques failed, yielding much lower and more inconsistent numbers. This suggests the initial high value was an outlier and that vacuum degassing the epoxy is a critical step for strength.
15:27 Exotic Methods are Impractical: An exploration of using plasma (gliding arc discharge) or a butane torch to raise surface energy proved too slow, dangerous (burning the part), or complex for practical application compared to simple abrasion or solvents.
21:26 Bond Prep Time-Decay Test is Inconclusive: An experiment to measure how bond strength decays over time after preparation was spoiled by messy, highly variable data. A mid-experiment switch from hand sanding to an orbital sander likely introduced another uncontrolled variable, making the results useless.
32:27 The Breakthrough - Consistency Over Strength: The creator consults an engineer from an orbital launch provider, who explains that for mission-critical applications, a reliable and consistent bond is far more important than a sporadically high one. The expert's recommended method is simple: wipe with acetone, then clean with IPA. No sanding required.
34:50 Final, Conclusive Tests: The expert's method is tested:
Acetone then IPA wipe (no sanding): Produces highly consistent results averaging around 3200 N.
60 grit sand + Acetone/IPA: Produces nearly identical, consistent results. The sanding step adds no discernible value.
38:16 Key Takeaway - Process Over Peak: The most important conclusion is that a simple, repeatable process (acetone/IPA wipe) is superior for engineering applications because it eliminates variables and produces predictable outcomes. The inconsistency in prior sanding tests may have been caused by contamination from the sandpaper itself, which the acetone/IPA process effectively mitigates.