Personally I wouldn't do plastidip. Just get some SEM (SEM-39143). It matches perfectly to the trim OEM sheen. You can get a can off Amazon for ~$10-15.
I think someone had a thread on here awhile back, but I just basically masked off the area using painters tape and a deck of cards to get clean lines. Then used some plastic sheeting to cover any overspray. Couple light coats and it was done. Still looks factory new ~2.5 years later.
edit: here's an example thread,
Who has changed the color of Front and Rear Valance
Just search for "paint valance". Lotta folks either do SEM, plastidip, or bedliner.