Novice needs help w/firewall chassis paint

[64skylarkls1]

I need some quick opinions/help. I'm doing an LS swap, have finished the mock up and now am in the middle of preping the firewall and frame for paint. I've welded in a block off plate for the heater and a few other areas. I am buy no means a body man but am a good fabricator and very fussy about getting things right.

As long as I'm at this point I would like to smooth the firewall. I've already spent way too much money and want to take care of this myself. My questions are very basic. I'm assuming I sand down the high spots - ridges of spot welds and so on, and then use a thin layer of filler to smooth out so....

To do this work, should the entire firewall be sanded to bare metal for the filler to stick?

This will be a rattle can job, what primers, paints are recommended for the firewall and chassis. I do not want a gloss finish.

I've done a few searched with limited results. Your quick response is very much appreciated.