How far do I need to cut to get to good metal to weld?

I'm glad it makes sense. Between the cartoons and **** running around in my head I'm shocked it's legible. Your xp is going to do 90% of what you desire. You may trip breaker on #4 power...a few 6" long penetrating welds on 1/4" thick steel and breaker will start tripping. For auto body you'll probably want power 2 with 30-35fpm wire speed. Start at 40fpm and decrease until wire isn't overfeeding. This will help prevent "blow through". It's time consuming but don't weld on the body more than 1/4" long bead at a time. On the area you're working there's probably alot of overlapping areas. Clean the areas between overlaps as GOOD as you can. Tight fitting overlaps will save your sanity. Get a box (25-50) of #6 screws and use a bit to make a hole that the screw can tighten fairly hard. I use multi stepped bits so the panel where i start my weld is larger than the other hole. Less welding and heat. If you have C-clamps or C-grips they make many jobs easier, cleaner, faster & professional. Anyway...once your piece is trimmed and fits well drill one hole then run the screw in and tighten it. Do several holes like that in different areas to hold your piece in to your liking. Now drill another hole 1/4"-1/2" away from each of those holes then tack those holes...mind the heat. Then pull one screw at a time and tack it shut. Ok...the short distance...1/4-1/2" is for intricate or smaller pieces. Say you're doing a quarter patch panel...those screw distances may be 12" apart (after the first 3 or 4 to align panel) then between each 12" span then between each 6" then 3" then outer panel hole then tack. If you'd like a few pics I can send or post them. I have exposed welds yet on this El Camino of different types. Damn I'm long winded today.