I share your dad's concern regarding velcro holding the skirts. Velcro will deteriorate over time, being exposed to the elements. You can't beat stainless steel 8-32 (or 10-32) screws and lock nuts.
You could look through
the Project Library for inspiration on various attachment strategies.
I'm glad you found a way to get your paint to stick to the coroplast. I cheaped out and used Krylon Fusion spray paint on my grille block.