The thing about just sanding away the edges is that underneath the outer layer of resin is a layer of paper, once you sand into it it tends to show up in paint. The paint soaks into the cardstock rather than covering it and you will have "funny" spots all over the place from the edges you sanded off, you could seal the edges again before painting it, but I still think you are better off with the method I use to end up with a molded look on top of a pep Helm...
When I am done, I hope people will be asking me where I bought my armor..
I am doing it by this method to get an accurately proportioned armor, and at some point in the future I plan to mold my helm...