If that Lincoln welder is an AC buzz box, stay FAR away from 7018! It's a reverse-polarity DC rod, and running it on AC is a royal PITA! Use 7014 instead, a 1/8" rod at 90-100 amps. It runs a bead on AC that's very similar to 7018, flat and clean, with slag that virtually falls off when you wave a slag hammer somewhere in its general vicinity.
If you want good penetration, use 6010 or 6011 for a root pass, and run a cap pass with 7014 or 7018, depending on the type of welder you have, to make it look good and add a bit more strength to the bead. I spent several years as a welder/pipefitter for E & J Gallo winery, using the 6010/7018 combination on black iron water and steam pipes up to 12" in diameter.
Jerry
