if you're doing a true restoration, the only 'permanent' solution for full thickness rust is to cut it out and weld in a patch. Surface rust, you need to sand it down and see just how deep it goes. if the metal is pitted, it's probably going to be pretty thin. I used to use rust converters for areas I couldn't reach easily that looked like surface rust. but eventually even if you add some por15 or other encapsulator on top, it will come back. it'll slow it down, but it will be back.
Now if your restoration is to get it looking good enough and useable for a few years, sanding, rust converter, then epoxy primer (some would argue por15 instead), some putty, 2k primer, and paint will buy you some time. You might end up repainting in a few years down the line with this method.
my 72 stepside frame off restore, I did a combo of cut and weld for body panels and some converter/por 15/epoxy for the odd shaped parts like the side steps and bed side 'rolled' areas. but for rust through areas even on the odd shaped areas, I still had to cut and shape some sheetmetal and weld. it's tedious slow work but worth it.