You control slag deposition in 3 ways…travel speed, arc length, and angle of electrode. Experiment with all 3.

Work on clean metal. It's not a deep penetration rod and therefore does not burn though surface contaminants very well.

In T-joint hold rot 30-35 degrees from horizontal plane, to make sure that enough is deposit on the vertical plane. Gravity will take care of horizontal plane

Having a slag stringer in the start of a 6013 bead is very common. What you are seeing is the heavy flux on top of the weld pool, and not molten metal. Getting a slightly slower start to the weld bead will solve this problem. A little hesitation before moving the rod will give the puddle time to bridge between both surfaces and solve this problem.

holding too long of an arc length, and it is difficult to manipulate the metal where to go. Put the rod right into the crack

6010

7018 (7016)