Por15 has there own self etch they say you can use over por & then prime, I've had trouble gettin stuff to stick even when using their tie coat primer.
Mines a few years old now & undercarrige is chipping down to the por 15.
So I'd probably sand the por15 to get a bite & then etch just to be safe, that stuff's like glass hard to get anything to stay, don't ask me how I know! Rich.