Hello ShowDownZero, you have a lot of possible variables that could be influencing the way that those rods will run or are running. Is there any possibility that you have misread the designator after the E7018 number? There are designators such as: -1, this is associated with an E7018 electrode that has met -40 charpy impact values, I believe(might be off a bit on that one), this should not be a factor for how you are describing the way that these rods run.
One possibility for a part of the spitting and sputtering could be from the excessive moisture that you say could have been picked up by the poor storage regimen, this could be compounded by possible electrical or thermal arc blow issues. A careful check of all electrical connections for best continuity might help to rule out that portion of a possible problem. Another factor could certainly be the condition of the generator portion of the machine itself, if it has a lot of corrosion and muck between the brushes and armature this could affect arc stability and smoothness. A check of the brush holders to ensure a good contact and correct pressure against the armature will additionally help to rule out possible issues, an additional check of the armature contact points for proper cleanliness will insure good continuity and machine performance. If the machine has a range selector or polarity selector there are additional points that should be inspected for continuity/corrosion issues. Amperage adjustment rheostats are yet another possible contributor. I think you get the idea here. A quick way to possibly determine if this is truly a machine issue as opposed to one with the electrodes is to take some of the rods and run them on another welder that you are confident in welding with, if the same results are exhibited I would look to the electrodes. As to your concerns with differences between the CC output of the engine driven welder being different from electrically powered shop sources, they all utilize CC current, the waveforms might be slightly different based on what type of system is producing the CC but it is CC all the same. You've got motor/generators(these can be driven with electric motors or combustion engines), transformer/rectifiers, inverters, and possibly others that I don't know of or remember that will produce the CC used for SMAW(stick) welding. You could run stick with CV as well, but it's not very commonplace. If you can try to include the exact brand of rod, also carefully check to see that your polarity is correct, I mention this because even though the machine might have the "correct" badging showing the correct polarity there are other reasons why this might be reversed. A volt meter set to DC current and attached in correct phase to the output studs will yield a voltage reading only, if the polarity is incorrect there might be a (-) sign in front of the numerical voltage reading indicating that the current is reversed. Hope this might help a bit. Good luck and best regards, Allan