It's okay but not really needed. You have to sand the POR15 to get anything to stick to it and by sanding it you remove its rust protection qualities.

Using POR15 Black on the inside of the fenders is the right thing to do.

Don't do the silver finish on the outside because the metal bits floating in the POR15 will give you fits trying to smooth it out. You have to spray over the POR15 when it's "drag tacky" to get the primer to stick. (wait for the paint to "gloss" then lightly drag your fingers over the area. When your fingers "stick" as they cross the surface without digging in, it's ready)