On the rear fender of my 124, there is a small dent just above the wheel well. has some small rust, which I'm going to fix w/ POR-15, but I was hoping to smooth out the dent while I'm at it. My dad says it can easily be popped out from the inside ... how does one do this? I don't want to hammer at it, because it might just make it lumpier in the other direction. What's the best way to do it? It's fairly easily accessable from the inside of the wheel well.

To do that properly is a skill that takes years of practice. My advice is to find a local Dent Wizard and have them do it. A local body shop should be able to help you locate one.