It is a very tough epoxy-based paint. POR is supposed to stand for "Paint Over Rust", though that is a bit of a misnomer. To really stop the rust, you generally need to use something like "Metal Ready" or Ospho or Naval Jelly. Then you can paint over that with the POR or similar paint to keep the water and acid out.
There's a website, check
http://www.por-15.com .
--DD