[Ant]

Whatever you use the surface will need reasonable preparation, more importantly is protecting the underneath of the tops of the guards that are prone to star crazing due to stones, missiles etc.

Some options:
1. Paint the area and its not that visible.
2. Use a lightish underseal maybe like has been mentioned in this post.
3. Also use some medium density foam glued in the main stone chip areas much like alloy body cars use e.g. Kirkham have it.
4. Make an extra skin (fiberglass) and epoxy up using urethane foam strips for some standoff like an extra skin mounted say 1/2" off the underneath of guard.