RE: can I mix 5 min with 30 min?
Epoxies get their strength from being highly cross linked. 5 minute is really only good for light service, 30 minute is best for firewalls, wing mounts, landing gear blocks. just let it sit overnight.
The polyester and polysytrene resins are simply long chain polymers of simple molecules. They are really only useful when reinforced with fiberglass since they are very brittle on their own. The organic peroxides used are correctly called initiators since they start the curing by breaking a double carbon bond in the monomer. Methyl ethyl ketone peroxide (MEKP) is the most common. Benzoyl peroxide was the first organic peroxide used in polymer reactions and is also used in acne medicines and for bleaching flour. I have seen MEKP in hobby stores priced at $2-$3 for a half ounce. I am retired from a company that makes MEKP and we sold it for about $2 a POUND. Of course our smallest package size was a 4 x 7lb case, guess the money is in the repackaging.