I have also heard this be called etching primer, but i believe its all the same thing? What exactly is it used for? is it ok to use it to temporarily cover up body filler?

 

Generally, self etching primer is thin, but eats itself into the metal better. Self etching primer is marketed to have a better hold on the surface then non self etching. A combination of self etching primer and filler/general purpose can be a good way to start your painting. Self etching primer is not for use on plastic.