if your re spraying it year after year, that means moisture is getting to the rust area, and causing the paint to bubble or peel or flake off,(and continuing to rust) with por that never happens again, it seals the rust and never lets air or moisture to get to the area again, thus preventing the rust from getting any worse than it already is.
don't sand it all nice and smooth either, wire brush it or scotch bright pad, (leave it rusty, remove anything loose) clean the painted areas around it and paint it on, that's it, very little prep work.
from what i've been told por was originally invented to coat battery trays, its acid resistant and all that good stuff.