Most "new" quality sheet metal has an E coat, Electrocoating. In the EPD family. This can be scuffed, cleaned and painted over. Some sheet metal can have what's called a cheap "transport coat" applied. It can be black and look like E coat. This type should be removed and primered.
It must be noted that there are OEM sheet metal parts that have real good Ecoat.
There are aftermarket, off shore, sheet metal parts with questionable "Ecoat" or unknown coat. (our parts)
This is where you get: "sand it, remove it, seal it, scuff it, hate it, love it" comments.
They are mixing apples and oranges.
Obviously since you only have one truck and are not a quick turn around body shop, you would be safest by removing it. Whatever it is. You will never know until it's too late.
On another related subject, you should not paint activated paint over rattle can primer. Lots of folks use rattle can primer because it's easy and protects until paint time. But it probably is not compatible with real automotive paint.
