the larger the rod, the more amperage you need on the dial. the more the amperage, the more damage you can do! the difference between 6011 and 7018 is twofold. 6011 has tensile strength of 60000 psi. 7018 is 70000 psi. 7018 is stronger if you can use it. 6011, however, is more penetrating. it will go deep into the joint. in any case, you should V out the crack. this will provide better penetration and a stronger weld. personally, i think 7018 looks better, if you do it correctly. one of the ways to compromise between the penetration versus the strength of the two different rods is to make the first pass with 6011. this will give excellent penetration of the base. then finish off with 2 or 3 passes with 7018. there should be a noticeable heighth to the weld bead. the height of the bead above the surface of the base should be as high as the base is thick. you are weakening the base slightly by heating it, so it needs to have extra weld material to compensate. 6011 is aggressive and you need practice laying down separate but overlapping circles that make up the bead. the 7018 is basically a drag rod. i wouldn't go any bigger than a 1/8 rod. you could even try and practice with a 3/32. every welder is different. play with the amperage setting until you have the right amount for any given size and type of rod. good luck.
