My Lincoln Classic 3D sat idle for quite a while, last time I used it the rods were sticking like crazy, I could barely get an arc started. Don't know if cleaning the armature is gonna fix it...
I never did it on a welder, but I have done it on an Onan marine genset. You need to know what voltage and polarity to use. If You are getting some output You probably don't need to flash it. Make sure the brushes are long enough and free in the holders while You are cleaning the commutator.