I generously brushed/worked Ospho metal treatment into the seams as best as possible. Gave it a long time to dry - even helping it a bit with a heat gun. Epoxy primer was brushed into the seams - not only around the outside but also any inside seams. The fender was spayed with epoxy primer and any seam or lap that looked like water could get into was filled with 3M seam sealer. Normal primer was then sprayed prior to the paint top coat.
It's been 5 years and appears to be holding. Yours seems to be in good condition.