I got a can of Bondo Fiberglass resin something something. Its green instead of the regular pink Bondo filler. I've used it great many times now on the lip and all you do apply enough of what you need and sand the rest of it off. When I first started using it looked like it wouldn't hold to what I wanted to do, and I was afraid that it would crack or simply fall off. Once its sanded level to the rest of the lip it really looked like a solid piece and it is. I've actually gone and hammered it (not to hard but enough to hit it to simulate a good size rock hit) and it withstood the shock. I would recommend to spread it out further out from the area being worked on so that there is some reinforcement.
I do have one area near the fog light that cracks easily (due to the lip being stretched to install onto the bumper), so I have to figure out how to keep it from cracking using the bondo and using more bondo to reinforce that area to keep it from stretching.
Also if you have a urethane lip, I have no idea if bondo will work for that. My lip is fiberglass and bondo works good. I've bondo'd my mp5 bumper due to a dent and it works ok, but that area is really hard but the rest of the bumper is stretchable (so far no cracks in the bondo'd area of the bumper). I'm not an expert on this stuff, but I think I've done a good job of repairs.
