Blender how to weight paint inside vertex or face for noob

Here is what you should do

while in the "WEIGHT PAINTING" mode, press "ALT B" and select part of your character then you can paint it inside

see screen below

this is part of the finger, while I press "ALT B" then select half of the finger then you can see there are still some blue color inside the finger, then I can paint it red so the WEIGHT PAINT is correct