Ok,
whats happening? See when you paint weight to one part it moves weights away from all other parts, when you remove weights from one part it puts wights on to all other parts. Now i can almost 100% certainly bet that you can not see what each paint is really doing.
Solution 1. hold other weights, this in general speeds you up to double speed at least. To do this double click on the pain skin weights tool to show up the tool settings, then go trough and hold all but the relevant 1 or 2 joints next to the weighted area that you want to share the values with. A few scripts exist to toggle this automatically speeds things up a bit too. If you do not hold weights you are almost certainly going to fail for N iterations, until the weight distribution is sufficiently like you want it after wich it happens to work out, off course in start that rarely is the case.
Solution 2. Make a dummy joint always hold all the unpainted mass there and unhold just the joint you want to add stuff into, and now you can also remove influences since all extra goes to the dummy joint, just make sure the dummy joint is empty before you finish and possibly delete it. (the dummy joint doesn't need to be real, you can just decide to flood all into left pinkie and fix that part last).
tip 1: try not to move back and forth too much do one bone pair area in one go then dont come back to that area.
tip 2: you can make a better starting solution by using influence objects to define the weights, leading into much less painting.
tip 3: put max influences to 3 or 2
