The glass cloth delaminated from the base and where the glue fillet was, it pulled the glass cloth away from the base where the glass cloth did not bond with the base plywood
First off, the laminate bond to the 1/4" ply plate/base never failed in any test including the one in the video. There were 3 layers of 6 ounce glass used. The polyester joints never pulled glass from the laminate with them. The epoxy joints consistently forced a delaminating WITHIN the fiberglass/polyester laminate. I did show the actual breaking of the joint and the joint after breaking.
I will say this, epoxy will stick to polyester resin with the right preparation. Sanding and Cleaning. But just using 5 min epoxy alone, your are flirting with disaster.
Not properly preparing any joint, regardless of the resin, is FLIRTINGWITHDISASTER. Every adhesive joint should be properly prepared before joining. Sanding the surfaces and cleaning should be common place even if CA is used. Amateur or not, this is how a good bond is created. If sanding is NOT performed the epoxy still bonds better than polyester but both are weaker. Often people use finishing resin when making polyester laminates. In this scenario the included wax floats to the surface and will mess up any bond. This wax must be removed with solvent and sanding to facilitate a decent bond.
The test did not differenciate if the epoxy was 5 min or 30 minute epoxy. Plus additives such as cavasil and cotton were added to the mix.
The epoxy resin used in the test video was Adtech 820. I've performed the same test as I showed in the video with Great Planes 5 minute epoxy and the 5 minute epoxy created a better bond to the polyester laminate than the polyester resin. You need to add fillers to epoxy resin like Adtech 820 and polyester resin to prevent the resin from running out the joint. If no fillers (cab-o-sil for example) are used then the bond is much weaker. Making a bonding joint with polyester resin and no fillers and fiber reinforcments (cotton flock) performs much worse than the video demonstrated. The bonding properties of a non-reinforced polyester resin is so poor that dropping it will break the joint.
If you will notice, the plywood tabs only left wood at the edges, where the resins created a fillet. There was no wood remaining in the center of the tabs.
A bond will always fail at its weakest point. In the polyester to polyester bond you find that the polyester resin used to make the bond fails. Very little of the wood sticks to the bond because the resin is weaker than the birch plywood test tab. In the epoxy bond you see that glass from the laminate was stuck to the plywood test tab. In this scenario the weakness is NOT the epoxy or wood but rather the polyester resin that was used to make the cured glass laminate.
Polyester resin is simply a poor adhesive compare to epoxy. Epoxy sticks to polyester cured laminates better than polyester does.
If the epoxy you are using costs less than $80/gallon in its bulk form it is probably pretty close to garbage. AKA what most people can routinely get their hands on. Likewise any epoxy that is a 1:1 mix ratio is also garbage.
Any epoxy worthy of the name epoxy, will have at least a 2:1 or greater mix ratio. Even 2:1 mix ratio epoxy is medium run of the mill stuff but it makes for easy mixing.
I will strongly disagree that 2:1 or 1:1 epoxies perform poorly. Hysol 1:1 and 2:1 bonding epoxies are used to glue full scale aircraft components together. It's higher strain and elongation properties allow it to perform better than most laminating resins in a bonding situation. MGS 285 is a 2:1 mix, has a Tg over 200 degrees (when cured properly), and is certified for full-scale aircraft construction. Many higher performance full-scale gliders are constructed with it. It is simply the best performing resin that I have used. It performs better than Adtech 820 which mixes at 100:18. Adtech 820 is a good resin just not as good at MGS 285. I never use the mix ratio to determine a resin performance. The Hysol 1:1 bonding resins still perform better than these excellent laminating resins in bonding applications.
Also, the price of the resin isn't always a clue to the performace. USC 635 is $71 for a gallon and performs better than many $100/gallon resins. See my test results here
. In fact, in high strain scenarios it beats the MGS at $230/gallon.