So sorry. I totally missed the notification that you posted.... like 3 months ago.
That thing is coming along great. Just watched your video and subbed btw. Definitely cool stuff.
Finishing is a pain in the butt with plastic. I print mostly with PLA and have yet to find the perfect solution. I have found that different filaments, and even different colors sand better than others. Filler primer works well in certain instances, but I mostly just use regular filler, then sand, then fill, then sand, then..... Ugh! That XTC3d stuff and also Zap Z-Poxy work decently, but again, only in certain situations. It would be good for your hard surface parts, but would destroy your fine ZBrush texture details. If used thinly, it will basically give you a thin coating that is a bit easier to sand than the plastic. But, gravity is a bitch. It will pool on you super fast and seep into details you want to preserve. In fact, maybe just ignore what I said altogether.
Anyway though, it sure looks like you've got it figured out. Again, great work.