rust treatments and the like are purely to prolong the situation, not fix it, the only sure fire way to remove rust is to grind it back, and cut out the bad material, and weld in fresh metal

etch primer is used on bare metal and welds, it essentially 'bites' in to the metal, protecting it from rusting away. high build is used after the etch process, it is much thicker, and builds a base for the paint coat to adhere to

the undersealing process seams to be a lot of personal preference, depending upon what kind of finish u want under there. anything waxoyl based will remain tacky and wnt dry completely, makes it better for rust protection, but can look shite and cant be overcoated. things like underbody schutz will dry and form an almost rubber feeling layer, but cant be overcoated so remains black. stonechip tends to be used in situations where the owner will then overcoat this with paint to match the bodywork. hammerite can be used directly on the bare surface, or also over the stonechip sealant below as a coloured overcoat