Hey Taz,
Welcome to the forum.
In my opinion using a 1/8" stainless steel rod on 3/16" is going to be difficult for anyone. Some of the manufacturers have flux systems that are more appealing than others, I just haven't run across one that would work with a 1/8" electrode and 3/16" thick material.
The reasons that SS is a little more difficult is due to the thermal conductivity. The molten metal stays molten longer and is more dependent upon the slag system to support the weld metal as it solidifies.
In addition, the joint configuration makes a difference. A tee joint will typically be easier than a butt joint. A gap of any kind can also cause problems.
I suggest 3/32" electrodes, start at the low end of t he manufacturers reccomended settings, keep the arc very tight. Manipulating the electrode from side to side with the majority of the time pent on the sides will allow the base metal to cool the weld metal better. That does not mean a weave bead, just oscillation enough to keep the heat spread out. I would also practice on thicker material and then work down.
Also, when your base metal gets hot, cool it off. When practicing, just cool it in water.
You can also practice on carbon steel using SS filler. The CS has higher thermal conductivity and will cool the weld faster.
Some other may have the key to the 1/8th on 3/16" vertical up. I have never been able to do it. 1/8" 7018 vertical up in anything other than a tee joint would be a little tricky.
Hope this helps.
Gerald Austin
weldingdata.com