Polarity on DC machines affects the bead height and penetration I think. You can accomplish the same thing by reversing the leads at the machine connections without even changing the setting on the dial. I do not have my reference book handy but going by memory, normal is the stinger carries the current and reverse the current carries thru the workpiece to the rod and makes the bead more flat and penetrate better. I think all the low hydrogen rods like 7018 recommend reverse polarity I am obviously not a welder by trade but I did go thru a short course on welding several years ago and actually still have a reference manual somewhere I think hmm I use a 220 volt Lincoln 135 MIG machine in my home shop with .023 wire and 75/25 shielding gas, lots easier to use than a stick welder. I never got real good with a stick welder it does take some practice no doubt. Miller makes good welding machines Lincoln also makes good ones the majority of the stick machines the sheet metal company I work for use are the old red lincoln 300 hi lo voltage machines the things seem to run forever. The majority of full size MIGs (probably 50 pc's) are MillerMatics Sorry not much help here on your question you got everything that I know about welding LOL