I think the self etch primer is supposed to be on the surface to make the POR15 stick better. Not applied on top for the next product. The etching part is referring to bare metal. Mine stuck just fine without it on car parts. The POR15 requires a topcoat of some other paint to protect it in certain circumstances, I seem to recall that you just apply it direct, the instructions on the can should clarify if it needs a scuffing to apply the next layer, your bottom paint. The POR 15 is so thin that I suspect scuffing it would expose bare metal underneath, defeats the purpose.