Rustoleum on your undercarriage ??

Have only used it (Duplicolor brand) on the inside of the door panels on an old pickup resto project. I was not particularly satisfied with it adhering to the surface which was POR-15. I would recommend you experiment with some different types of primers on a test surface until you find one that will bond with the bed liner. As far as using the stuff on the underside and fender wells, I personally prefer the smooth finish like modern vehicles have. Deposits of road grime etc. tend to stick to rough surfaces but rinse off quite easily from smooth ones. Who hasn't found rust under old undercoating? I am using Rustoleum primer followed by 2 coats of Rustoleum oil based paint and getting a nice shiny tough finish in the fender wells of the pickup. Time and miles will tell.