I agree with Yakimabowtie. Body plastic is designed to be flexible enough to stay put on running boards IF mixed correctly and applied to a clean rough surface. Back in the mid '60s I worked for a Chevy dealership in Kansas City KS in the used car body shop. We used a LOT of plastic

. The boss had me do some patching on the sales office concrete steps. In 2010 on the way home from our vacation we stopped by the old dealership site. The dealership has burned to the ground in the '80s but the used car shop & sales office was still there. My patching was still holding & in good shape.