Looking at the various high spots/peaks showing on the outside, it appears there still exists quite a bit of pitting on the backside that, although has not broken through yet, the sandblasting from the backside highlights the extent of the pitting damage to be quite extensive. I think to fix that particular fender, it would require new metal in order to be a more permanent repair not requiring attention a couple/few years down the road. Looking at the amount of work the repair will require, I think realistically you should look for a new fender or a solid used replacement.
