well filler primer is like body filler in a spray can it sprays on thick. It is used to fill scratches and small dents. regular primer is more like paint. i use both i use the filler primer if i over do it with coarser sand paper, it fills in the deep scratches. when I get into the finer coarse sand paper i use the regular primer. i also alternate primer colors so that i have some reference how deep I'm sanding down in to the layers below. this also help make sure you have primed everything by giving you a color difference from the layer before and showing you where you have missed a spot.