Flexible primer?

[FY1Cuda]

WE got two elastomeric bumpers for our '70 Cuda from the guy who makes them and sells them on Ebay.  He cautions to be sure to use adhesion promoter before painting them, but there is so much prep to get them smooth that we needed multiple coats of surfacer/primer to get the bumper ready for painting (one of the bumpers had no wrapping inside the box, so the big copper staples scratched the crap out of the bumper).  I asked the guy at the local paint store (a Finishmaster) and he assured me that a urethane primer has plenty of flex yet it is easy to sand.  He sold me a quart of Omni urethane primer and the hardener.  Now all the thinner sections of the bumpers have cracks in them (after getting their finish paint jobs).  I went online to check about this primer, and the website mentioned that if you're putting it on a flexible surface, like a plastic bumper, you add the Omni flex agent to the paint.  Of course, the paint store guy didn't tell me that and doesn't normally stock the flex agent.  So...
What kind of primer can I use to surface the rubber bumpers so they don't crack?  The finished color of the bumpers is FY1 Yellow, so I need a white surface to paint over.
Any knowledge on the subject is appreciated.

[DocMel]

Flexible primer, if needed, wont do a thing unless the actual paint on top of it is flexible as well. 

Actually, paint that might need to flex a little is just paint with, you guessed it, a flexitive added to it from another can:   No big deal

Keep in mind, flexitive added to paint is really only required for areas that would tend to flex:  Think of those thin plastic filler panels you see on some cars between the bumper and body.  Your bumper will NOT need flexitive wether its in the primer or in the paint, unless you plan to twist it for some reason,  Even then, flexitive will only go so far to prevent cracking

Think fiberglass hood.  They flex all the time, but not to the point were the flexitive is needed. But for peace of mind, you can add it if desired in most paints and primers/clears:  Its not that expensive

[FY1Cuda]

I was led to believe that the urethane paint was flexible enough without the flexitive, but you raise a good point. I know that the primer had cracks because when the painter top coated the bumper, there were raised lines over the cracks.  He sanded those areas down, primed them and painted them, and I thought we were good to go.  We didn't see any cracks (or raised lines) in the finished bumper until we were very gently installing it, and then we could see tiny cracks in  the corners. The bumper is very rubbery at the ends--the outside edge that hugs the fender is just a little more than an eighth inch thick, but we were hardly moving those areas.
Anyway, thanks for the info.  I'll check about a flex agent for the urethane top-coat.

[DocMel]

The cracks you mention sound very much like either surface prep contamination or one layer of something is reacting to the other

I dont beleive flex is the prob:  If its surface or some sort of chemical/product combatbility reaction, the only cure is to COMPLETELY remove all previous coats of what ever is on the bumper and start over, ensuring anything that is used is compatible with each other

[elitecustombody]

Flex agent or flex additive.I never heard of "flexitive" .

OP, using Omni on anything that's worth than $1000 is just wrong,especially when it comes to primer. If you want great flexible primer  that can take abuse,try Sherwin Williams P30, they come in variety of colors,gray,white,black,yellow,e.t.c. If you plan on using flex agent,make sure to use it in primer,basecoat and the clear,otherwise you will still end up with cracks if one of the does not have flex agent mixed in. You will have to at the least remove all material where the problem areas are,covering them up will not work.

[FY1Cuda]

THANK YOU.  That's just the kind of info I was looking for: a recommendation for a particular product from someone who's actually used it.  We have a S-W auto paint dealer about a 1/2 hour away, so I'll get on it.  Thanks again.