Okay, here's one for the fiberglass-experienced folks.
In the past, on a few occasions, I've tried to 'glass or glue steel or aluminum to molded fiberglass parts, with a 100% failure rate. I don't know if it's my surface prep, remaining mold release agent, or what, but despite cleaning with a battery of different solvents, sanding, etc., nothing ever sticks.
Now, at this point, I have these spectacular fiberglass front cycle fenders for my Locost, made in the UK. They're made of polyester resin & glass, but I don't know the chemicals, mold release agents used, etc. Because I don't want to drill & bolt through those glorious fenders, my plan is to make up inverted boxes of 20 gauge steel (curved, using my new stretcher/shrinker set) which will bolt to my steel fender stays. The closed faces of the boxes (facing up) would be glued to the underside of the fiberglass fenders using 3M 5200 (the hold strength of this stuff is astounding, and it works even better when exposed to water - Cat***ham uses it, or something like it, to attach their cycle fenders). The shear strength of 5200 is apparently 362 psi on fiberglass (much greater on steel). As the steel-to-fiberglass bonding area will be roughly 42 sq. in., the grip strength would be a little over 15,000 lbs. (7.6 tons). And that's just per fender!
My concern is that if I can't get the undersides of the fenders cleaned sufficiently of mold release, etc., the joints could fail (not the glue, but the interface), and I could lose a fender.
I have on hand various soaps, paint thinner, isopropyl alcohol, acetone, xylene, etc., in addition to various sanding products.
Any suggestions as to a sure way, or series of steps to follow, to ensure the bond as secure as the glue?
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