actually bondo brand filler is fairly hard to sand compared to others for sure. I would recommend getting some automotive grade filler, it will be way easier to sand than the bondo brand filler. Only way you will get your armor smooth, and it works perfect. here this is pic of my build using auto grade filler all the way.
so far almost everyone on here has given you a correct answer, I use bondo brand myself, but I would love to see the brand that you use Finhead. If I remember you do all your work by hand, where I do most of mine by machine (sanders, dremels, etc.)