why is my welding inconsistent?

I would say power is too high and you are moving too slow. Wire speed will cause splatter and popping. Turn the power down to about a third of what your machine is capable of. Adjust the wire speed until it sounds like a consistant bacon sizzle. I also like to keep the tip at a 30* angle to the welded surface and push the weld not pull like the old stick welders.

If you are installing a trunk pan you should be using plug welds. Then seam seal all the joints.