I don't have the 16" portawalls, but remember the old "Jakester" whitewalls....I even used it to keep the Galaxie whites looking nice.
Mask off the edge with the good blue fine line tape. Clean the area to be painted with lacquer thinner. 2 to 3 coats of SEM white vinyl color spray bomb available at most auto supply stores. There's so little flex in those space saver spares that I doubt they'd even crack.
I assume you're using them on a trailer?