You've touched on one of my
You've touched on one of my gripes. Our *first* Civic, a 1981 model, had much different bumper. Going from memory , best I can recollect, the bumper was chromed steel, with a wide black rubber contact strip continuous along the front face and through the wrap-around corners. You could actually "bump" into things, say give another car a push, or whatever, and it would come out more-or-less pristine. It's main mounting to the car's frame was with some kind of hydraulic pistons.
During the '80's the speed rating requirements for bumpers dropped (now who might have asked for that?), and bumpers evolved into black plastic fairings, with the higher end models painted in matching body color. Of course, the slighest ding showed the black under, and their name became an oxymoron. In any sort of fender-bender, the "bumper" was usually crumpled, scraped and/or dislodged from it's cheap attachment points to the frame. Usually the frame got buggered up at the connection points.
Anyway, rant over, you must be driving where there is some gravel kicking up, to be accumulating so much. Even if you aren't, todays bumper systems are for the most part fragile jokes. If you parallel park regularly you quickly collect "kisses" on both ends, from braille-method parkers.
I'd say just let them accumalate, hit them with the touch up paint dabber, till you can't stand it, and then get the bumper removed and repainted. If you can handle removing it yourself and getting it to a painter, you might save a few bucks. The shop manual is good for this.