Not sure what you are talking about when you say “using the paint tool”
There are far better ways to whiten teeth though…
Here is my method of choice:
-Turn the magic wand into quick selection tool and select the teeth. You can select them however you want though
-copy and paste the selected teeth
-set blend mode in your layers pallette to screen
-mess around with changing the opacity of new layer, and also using image adjustments on the new layer as well.
There are seriously 30 different ways to whiten teeth, and it would be beneficial for you to try out a few.
You can do some creative dodging, you can use a paint brush with a low opacity, etc… Search on google for teeth whitening tuts.
Good Day