I might suggest that you grind the plates, to remove the mill scale, prior to welding. To illustrate the benefit, I like to have the welder grind 1/2 of a ~6" long plate and then just run a straight stringer bead down the full length. When you transitiion from the unground to the ground areas, the bead will wet out and look much better. It is really obvious when running stick electrodes, like E7018, but makes a big difference with all welding processes, especially with GMAW in short circuit mode.
I agree with the previous comment, in that you need to make a more continuous movement with the torch.
Both suggestions will aid in assuring adequate root penetration.
Good luck.