Never heard of anyone actually taking a tank apart and welding it back together again. Usually they put some gravel, or old nuts and bolts in it and shake it around good to knock all the crud loose. This can have varying degrees of sucess.
I guess if you plan to open it up, the first thing is to make sure it's rinsed and re-rinsed so that there are NO fumes left in it. Otherwise, one little spark will send you into the next world!!
