I recently was doing almost the same project as you. I replaced the stock bumper cover/airdam on my 02 xtreme blazer with an aftermarket. The aftermarket bumper cover was made out of urathane(sp?) vs. the abs that the stock one was made out of, so the new one had quite a flex to it. I picked up a can of OMNI flex-add and you mix it directly into the paint then spray the part. In my opinion you really don't need the flex-add unless you're seriously going to be manhandeling the painted parts before/during installation (as I did to install a mesh grill and fog lamps). Peace-