Can you take fibreglass hardener on a plane?

No, because it contains a ketone peroxide. DOT classes that in "forbidden materials": Electrical devices likely to create sparks or heat, heavily magnetic materials for air shipment, incompatible materials in the same package, materials that may undergo self-accelerated decomposition, ketone peroxides, etc. Next question: why would you want to? Every Lowe's and Home Depot in America sells it.