I can get my hair really straight with a round, boar bristle brush. First I apply a leave-in and then some Phystodefrissant. I blow dry with just a vent brush until it's mostly dry, just a little damp. That's when I begin blow drying in sections with my round brush, which smooths my hair out. Usually doing this prevents me from needing any type of finishing step, no serums, no flat iron. Honestly doesn't take me much more time that diffusing curly.
Fine, blonde 3a botticelli curls