I am far from an expert, but my understanding is that the cf is manufactured to comply with Cal. voc standards. The kp21 is what hok recommends for most applications. I've only done two paint jobs, and used kp21 on both. I had no adhesion problems at all over bare steel or fresh fiberglass gelcoat. It was a little hard to sand, but both times were on motorcycle parts, so I didn't mind. My thinking at the time was that it was supposed to be a little harder to sand, providing a more durable foundation for the color coats that followed. I'll use it again on the next one.