I am learning to rig in Maya [2010] (for game design). I created a
skeleton, smooth binded it to the model, and now am painting skin
weights.

The problem: Every time i paint a skin weight, and
then go to another weight and paint on it, when i go back to the 1st
weight, it has dramatically changed! its completely different! so
basically all progress is removed when go on to the next joint.

here are some screenshots showing what i mean

Yes that would indeed be meant to happen happen unless you locked off all OTHER joints so maya knows where to put/ take the rest of the weight from. See there just has to be weight and the extra wight has to go somewhere. No if you paint one joint black the data will go to all the other nonlocked joints equally distributed.

Wieghts do not disappear they stay constant, the wight is shared amongst all the remainding joints. This is why its often not so productive to erase weights just add as its more predictable the thing what got taken away then what got put in. Then flood one joints with everything and start adding away in others.


Incidentally you want to use rgid binding here, as you dont use fractional weights.

I can't figure out how to paint skin weights on a rigid bind.

why you dont try using the skin weight tool from this site. It can help you to assign polygons directly to the bones and in that way make corrections more easily after you painted the skin weights.