Danae, as the not-so-proud owner of super baby fine 4a hair, I can tell you that nothing will lock out 90% humidity. Our hair is so susceptible to any level of humidity, it's not even funny. And, my hair wouldn't hold a curl either once I did get it straight. Baby fine hair is the most difficult to deal with. I BKT now, and I no longer have problems with humidity, but I don't wear my hair straight. But, it is super easy to straighten now, and it stays straight--too straight, in fact--irrespective of humidity. But, it's so soft that it won't hold a curl from a curling iron at all. Good thing I wear my hair curly--it's about a 2b curl pattern now. I only wear my hair straight in the waiting period right after a BKT application, and it stays straight no matter the humidity.
Even spraying hairspray on baby fine curly hair makes it frizz. Trust me, I've tried it all.