Sorry, this is probably a noob question, but should epoxy primer be laid down first, onto the bare metal, or is it safe to spray on top of self-etching or even filler primer?

 

It's fine to spray epoxy over self etching. But self etching is mainly to slow down rust before you lay down your filler or other high build primers. If you do have a rust issue I would start by eliminating the rust issue ahead of time. Self etching primer may slow down the process of rust but it will not stop it. Self etching primer has phosphoric acid and zinc in it that just slows it down.

I guess where I'm going with this is if you have the need to use self etching primer I would evaluate your situation a little closer and deeper. Possibly scab in a new panel.

But to answer the above question, yes. You can spray over a self etching primer with an epoxy primer.

 

Epoxy will work on any surface you need to read there P sheets to see how to get the product stick to the old surface and what sandpaper to use. I would say 80-180 grit will be needed i order for it to stick.

 

This is not true. You should not lay down filler over any kind of primer (fillers should be direct to metal. Only fillers specifically designed for application over primer may be used over primer.)

Self etching primer acid-etches the substrate (hence the term self-etching.) Self etching primer is used to provide mechanically and chemical adhesion to the substrate before applying the surfacer. Most surfacers are NOT direct to metal and MUST be applied only over self-etch or epoxy.