Mine's a '71, and originally came with a urethane bumper, but I converted to chrome because my painter couldn't figure out what flex-primer is all about. But, that's another story for another time.
Hey Eric,
I never used flex aids of any description on refinishing my '73 front bar, and it's all AOK!
Greg.
My bumper needed some divots repaired, so I tried my hand at that - they came out great, but I gave it to them with the car, and his body guy obviously did nothing more to it before he shot primer. I'd used some aggressive sand paper in a vibrating palm sander to remove the paint - which I'm thinking I maybe should not have done. I'd also used some "Icing" body filler to fill the divots, which I later also discovered I probably shouldn't have done (used a different 'more flexible' filler & technique).
Suffice it to say, with the aggressively rough surface of the urethane, the primer didn't go down properly - it cracked as it was drying... every place except for the repaired places I'd used the more stable Icing, that is. I was not anticipating the much more knowledgeable than I paint & body guys to accept my repairs as "finish-level" and actually start painting the piece.
At that point in the game, I just took the bumper home and ordered the chrome replacement I currently have on my car. I'll be doing some better research and re-repair the bumper properly before I repaint it. Once that's done, I may toss it back on the car if it doesn't take too much adjustment of the repop fenders and valance to make it work.
I think the urethane of the '71-'72 bumpers might actually be a little 'squishier' than the '73s as well. I remember noticing that mine had significantly more stress fractures in the factory finish than Auron73mach1's bumper. Of course, that doesn't mean a whole lot considering the age and conditions of storage for each of the cars.
Honestly, once I'd resolved to going with the chrome bumper, I kind of even like it a bit better than the stock urethane bumper - it "matches up" the rear end better, and works well with the chrome rims and bright work trim around the glass. Gotta have chrome on a classic muscle car, after all.