Backyard method, but it shows the flange pretty good. I did not have a power flanger, used a hand tool, much slower but gets the job done, borrowed it, but eastwood has em, not too dear.
As far as protecting from rust I used a tube of Por15 seam sealer(a thicker version of the paint) crammed into the seam in the back, my lip is upside down, so hopefully no water would lay in there if I missed any spots? Rich.