That's looking really fantastic!! I'm not a fan of Bondo (or anything toxic, for that matter), and I'm really surprised, happy, and impressed with how you're using the combination of pep and clay. After seeing the results you're getting, I'm super excited to try this out myself! One thing, though, I would recommend smoothing it out more, if you can, as parts of the surface still look a bit uneven. Fill in the low spots and scrape down the high spots. What tools were you using for the smoothing? Did my noob-tips help you at all? Admittedly, they may not be the most helpful, but if you work in small sections, take your time, and have the right tools and good lighting, it makes a huge difference. If, however, you're planning on working it out in the cast, I can understand that, too... smoothing clay is hard. I'm sure you'll let us know how it goes!
What are those gray lines, by the way? Is that just the pep piece showing through?