This is only my opinion; filler would work on running boards, but I would project that the constant flexing imposing by people stepping on the running boards would eventually disturb the filler. My truck originally had fiberglass repairs made on them. It seemed OK, but very ugly. I decided to get replacement running boards because I felt it was a "high traffic" area, especially since with the four wheel drive installed, the truck sits much higher and you have no choice but to use the running boards to enter the cab. Again, just an opinion.
