The advantage of etch-primer is that it chemically changes any remaining rust (only a very thin layer is required), zinc-phosphate primer is just an adhesive unlike the zinc paint we used to be able to get.
Like to sand out as much rust as possible, but there is always a chance of the odd speck re-growing.
In a time of universal deceit - telling the truth is a revolutionary act. George Orwell
When the debate is lost, slander becomes the tool of the loser. Socrates