rust prevention

take the fender off.

grind it all down around the spot

find someone to weld a piece in for you.

when done, get the inside mostly flat. degrease it. apply the por 15 metal prep. apply por 15. then undercoating.

on the outside get it smooth. epoxy sandable primer. re smooth. re prime. re sand.

have someone shoot some paint on it.

fix it one time. done.

if you are going to all that trouble, with just treating... you might as well fix it right. in order to just treat it properly you have to take the fender off anyway...

really undercoating dont do crap por15 on the back with undercoat over it. but that requires getting the back ready for por15 and you need the fender off.