How "porous" is red primer?

Sometimes I like the look of RustOleum red primer (usually the rusty metal primer) and don't use a top/color coat. The first time I did this was back in the '70's when I painted an 8 3/4 rear end before swapping it into my '65 Valiant. I also used the same paint for the interior cast surfaces of my bracket race motors. For the five years I kept the car after that, the primed parts held up almost the same as the day they were painted. I am still doing this on certain parts because I prefer the red oxide look or because the parts are hidden from view.

Last night, I was reading an article on the internet that mentioned how primers are porous, and now I wonder exactly what that means and what the long-term effects might be. Maybe this "porosity" was aimed more at normal body undercoatings and not the high pigment content of the RustOleum I use. Comments?