It really depends also if you have had metal repair done. POR15 isn't just enough in most cases. Maybe for real light surface rust.

If you cut out the metal and don't treat the metal properly (reason why metal repair doesn't work 90% of the time) before welding new metal it will only come back again like cancer. All of it has to be cut and fixed and treated properly. POR15 is simply a bandaid in most cases. Especially against Canadian winters.