Primer Sealer and Red Oxide question

Years ago, a friend brought me his 67 Riviera to paint. It had a combination of light gray, dark gray, & red oxide primer all over it. He wanted the car painted in the factory light blue metallic & I told him that unless I either reprimered it, or sealed it to a solid color that I wouldn't do it. He found someone else & 2 weeks later he brought it back to me to repaint. The paint job had spots where you could see different shades because of the different colors of primer under the paint. I don't do much painting these days, but if it were mine I would either primer it one color & wet sand it or put a good heavy coat of sealer on it & then wet sand it before final paint. As they say, "A good paint job is 90% prep work".