Pep files are pretty hard to use for armor. I must say though, you're doing a pretty bang-up job at it. I used wire for some of the edges to strengthen it. I also fiberglassed the inside of the forms, but for a different purpose. At some point, you'll have to bondo (or whatever the equvalent is where you live) to smooth out the outside of the molds/forms, so I wouldn't build up the fiberglass too much on the inside. Fiberglass is somewhat hard to work with to get smooth if you use it on the outside. And I fear that your armor will weigh a ton, compared to ABS. Not to mention, will end up running about the same price. You'll also want to keep it as porpotionate as possible, otherwise it will look a little wierd. My biggest challenge was to get the piece I built to match the size of the other things I made.
The bucket is looking really cool. You might concider buying a ABS helmet, because the fiberglass might be a bit heavy over time, sitting on your head. Foam padding will help, but I can't help but wonder what that much weight will do sitting on your head/neck for extended periods of time.
Good luck with the rest of your build!