For my frame (I only did the part behind the cab, as i had the bed off) I just took a wire brush and some sandpaper on a orbital sander for the flats, and it worked pretty good. Theres pitted areas, and it didnt look the best in some places, but you couldnt even see them when the bed was back on.
I painted it with some gloss black spraypaint, and its already not so glossy (this was over the summer).
My friend did his frame with just rattle can paint and its held up good for quite a few years so i figured a good few coats should make it look nice.
Paint it with a semi gloss or satin, as thats what itll end up as anyways, dirt roads and mud (not even mudding, just rainy days) will fade it real quik, it looked amazing for the first month or so haha, but if you dont want to spend a ton on the abrasive products just go wire brush and sandpaper, great results for me