best rust converter?

I swear by POR 15 paint, but either spray on your primer while it is still tacky or use their self etching primer over it after its cured. Personally, I always spray the primer on while it's tacky and have always had great results.

Smaller areas of rust, I use the Mar-Hide rust converter. It comes in a spray can and is fast and easy to use. Any paint or primer sticks to it when it's dry.

I have a HF el cheapo wire feed welder myself but haven't tried it on body panels yet. Great for everything I've used it on so far.