POR-15 works great until it fails.

The directions say apply to rust for best adhesion.  Unfortunatly, rust is
not attached to the metal very good.  Reports from others, it works great
until it peels off.  My simple test had it fail and peel off.  I blasted and
left a very light coating of rust and painted.  One hit from a hammer and it
all peeled off.

I never tried zero rust.

The question is how long do you want it to look good?

If you want more than  a few years you need to take the rust off.  Spray a
good epoxy primer and use 2k paints.

Have never had anything but good luck with POR-15.