Oh didn't know that those were natural finish bumpers. That does make it hard to touch up. The line-X idea is kinda cool. I've seen a nissan frontier pickup that had the plastic front bumper cover done. Looks good.
The product I use is a two part adhesive that dries hard and you can sand or grind it. It's called Norton Speedgrip Structural Adhesive. Lord Fusor is another brand. They have different working times from 1 minute to 45 minutes and it's about $15 a tube. Any auto part store that sells auto paint should have it or if you can find an automotive paint and body supplier they'll sell it too.
I haven't had luck with any of the regular body fillers and even the ones with chopped fiberglass (bondo, evercoat, etc.) filling gaps. They all eventually seem to crack and fall off. If you line-x the bumper afterwards though that may be enough to hold it together even if the filler underneath cracks. Regular paint wouldn't but the line-x may.
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