why is my welding inconsistent?

Yes panel thickness will vary a bit due to rust etc, also as you weld you carry more and more heat as you go. The method i use takes a little longer but i use it for 100% penetration (looks like a bead on both sides), and to eliminate burn through which causes more work etc. I run my welder a touch on the cool side, not so cold as to get a cold lap type weld, slightly starved on wire speed (too much wire can push right through the puddle) to compensate on thin sheet metal, and start and stop as the puddle dictates. If the puddle cools reasonably fast, stitch it again. If its hot wait for the orange glow to fade a bit and off ya go etc. I prefer to pull the weld on sheet metal, gun angled toward the seam welding towards yourself. Its easy to see the puddle this way. With a little practice you can watch the puddle and know when to stop/start. You want to be stitching before it cools too much or it'l pile up each time. I can use either .023 or .030 on 22ga steel with just a small change in heat/wire speed. Solid wire with gas is the best route to go, C-25 will give you more control and less spatter than Co2.