7018 is a 70,000 # tensile strength material all position stick electrode usually designed as a LH rod for low Hydrogen flux to prevent inter granule cracking. For first pass, try 6010-5P (red wire) for better penetration, then use LH7018 cover pass electrode OR 7028 JET WIRE if appearance is important. 7028 should only be run in flat horizontal beads. However, if using MIG, ER70s electrode wire using either 100% Carbon Dioxide gas or C25 gas that is 25% Carbon Dioxide and 75% Argon mixture, is a good choice.
BTW mild steel is considered a low carbon steel ok for fence posts and non critical applications but 1020 carbon steel or 1045 carbon steel for higher strength applications may be better. Thd higher percentage of carbon adds strength noth in tensile and bending.
BOB RENTON