Help with a 1974 Trans Am front bumper
 

My 1974 Trans Am is almost finished, but we are running into a problem with the black section of the front bumper. The car has only 49k original miles and has been stored inside almost it's entire existence, hence, the bumpers appeared to be in very good condition. Some minor cracks were filled by V'ing them out and then using a flexible primer. The problem we are running into is that the bumper keeps getting cracks if it is flexed.

Is there a way to prevent this from happening again? This will be the third time repairing this bumper. I want to make sure we do it right. What is the shop doing wrong?

Pic of bumper cracks

i posted on your other thread on this but wanted to post here too. im sure some other experts will chime in--

is it an endura material like the earlier 70-73? if so or even if not, confirm what filler they are using on it & if its not the 3M or SEM flexible bumper/plastic filler thats probably why its cracking. ive used the SEM flexi-weld product on a few different materials from a 72 endura nose to a 78 nose & also 79-81 flexible wheel flares, its held up crack free for 10 years on the 78 & 81 cars & going strong on the 72 nose for 5 years. this is in the midwest where the cars go from 0f to 90+f each year as well as lots of flexing when installed.

another thing guys with endura bumpers do is to use a coat or 2 of epoxy primer, everyone says API is the best but i used a different brand on the 72 nose & it likely helped a lot with avoiding cracks.

& a final thing to try if they arent is a flex agent in the paint/clear on top of a flexible primer, flex agent will cure out but allows the parts to be manhandled during installation without cracking as easy... on my 81 i had some welting left over that was painted with flex agent, could bend it almost 180* & it wouldnt crack, a couple months later it would crack at much less of a bend angle.

hope you can figure it out but need to confirm what material they are using, painting plastic & rubber can be a B!

I will get more details on the products and process they used.

Whatever they are using, the first step is to open the cracks up with a Dremel or something like that. No filler will get into those hairline cracks.

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I have repaired those. These cracks are not deep. They are in the "sealed" coat or some may use the term "gel coat" but its not a glass bumper strip. Looks like yours is on the inside?
To fix it, you'll need to blast it with plastic and get it down to the more rubber type body. Once you do that, use SPI EPOXY PRIMER ONLY. prep surface with waterborne, then solvent based cleaners and follow the free directions they send or use their forums.
You MUST wait 30 min Plus ( I use 1 hour) or up to 4 hours of Induction time ( mixed and awaiting being sprayed) the longer the paint is Inducted, the more UV protection it has.
Put on primer, wait 30 min( use a timer, don't guess) and put on 4 coats.
Wait a day and start to block it with 220. This primer sands well and REMAINS flexible. You can apply a "bumper" black type paint on top of the primer.

I lightly sandblast them and then build them back up with multiple rounds of SPI epoxy and block sanding. Takes a while but works great. They look pretty rough after blasting but all the cracking material is gone.

Don