if you use regular bondo and resin, i used something like 75% bondo 25% resin. If you plan on using fiberglass with it, dont put the mat down first. I've heard that rondo doesnt soak in to fiberglass. Instead slush a layer of rondo in the piece and when its still wet, lay fiberglass mat in to it and cover it with some more rondo. It's called mudglassing. That's what i did to my chest piece on my Mk V build and the pieces are rock hard!!!