as said body filler shouldn't be laid on that thick, the metal should be pulled out close to where it should be and filler applied to smooth things out. hard to describe in type, a few tools that are common are a nail gun, basically a small spot welder that tacks copper(I think) "nails" to the panel, then a slide hammer to pull on the nails. a slide hammer is basically a pipe with a chuck on one end to hold the nail, a weight in the middle and a stop on the end, you slide the weight back to pull the dent out. you may be able to rent these things, not sure. for prepping to do that, the panel needs to be ground to bare metal. when you have a panel close to straight, then load up and spread your mud, when it's no longer soft, start sanding with 80 grit on a block, or if you put alot on you can start with a DA sander or air block, like a hutchins. keep filling low spots, finish with 180 or so, prep the paint around the mudwork with 220-320 on a DA, then prime, block primer with 220 or so with a hand block use a guide coat(a dark color paint with light colored primer, black in a spray can works well) to find low spots in the primer, wet sand with 400-600 use guide coat again to remove all block scratches. then use a grey scotch-brite(wet) to scuff the rest of the panel, and clean everything! before mud blow the panel clean as a minimum, wiping it down with a wax and grease remover is a good ideal, in between coats of mud, blow out all the dust well, before primer use wax and grease remover on paint and metal around mudwork, and wipe off with a clean cloth, also I like to wipe the mud with a clean cloth as well, after primer is sanded would be a good idea to clean the panel to be painted with soap and water, use dish soap, as some car was soaps contain silicone/wax/other things that paint doesn't stick to. wipe entire panel again with wax and grease remover and wipe off before painting.
I hope I've been somewhat helpful
good luck if you should decide to go through with this, I like RM 900 pre keeno for a wax and grease remover btw, any other Q's PM me and I'll try to answer them, and yes I paint for a living
