Personally, I think weatherning is the easiest and most fun part. Just paint your nice metalic color after a filler primer, then put on liquid latex. You can find this in art stores. The application is the fun and simple part. Take a pen, dip it in the liquid latex and then flic it onto your armor. Make interresting patterns,put it on ridges, corners, etc. Any part you think the paint would get chipped off. I put a layer of paint that is similar to the final coat on to simulate dirt or something like that. Keep in mind that spartan armor is made out of titanium I think and titaium does not rust. Once that coat dries, peal off the liquid latex and then reapply it one the silver in an irregular fashion. Do not fully encompass the area. After that, apply your final coat. For a really deep intricate look add a very thick layer of your final paint. Peal off the liquid latex and your done. As you wear your armor, the paint will chip and the armor will damage, as this happens you will notice that you already have a nice compliment of scars and burrs on your armor, just paint silver withing the lines of your paint as it is damaged.
Now I think that it's not a bad idea to make a new Spartan but imperfections in the bondo process will be very very visible. Thats one thing to consider. One thing to consider also is to follow the steps above, except paint your final coat over everything without the last liquid latex process. This will provide the appearance of layers with a fresh coat of the final color. So essentially, you just painted over your battle damage.
Hope this helped.