With the body sealed, it's time to load the body up with a good filler primer. Filler primers are very heavy and add thick layers of micro bits of "body filler-like stuff" to block-sand off. Notice the spotty black streaks all over the panels. This is guidecoat. Its basically just paint applied in a very light coat over the entire surface to be block-sanded. As you sand, the low areas will still have the guidecoat visible, thereby letting you know that additional filler or primer is going to be needed.