You can create a mask on the mesh that lets you paint specific vertices only, blocking all other vertices from receiving weights.
To create a mask while painting skin weights
Tip:
To toggle between
Paint Select and
Paint modes in the following steps, use the hotkeys
+ > and
+ < to change modes to the right or left.
- Select a smooth bound mesh and open the
Paint Skin Weights Tool.
Note: The Paint Skin Weights Tool is not available when skin layers are present. Use Skin Tools for painting weights.
- Set the
Mode to
Paint Select.
- Paint to select only the vertices you want to keep open for weights, then switch the
Mode to
Paint.
-
As you continue painting weights, only those vertices you selected can receive weight values, and all other vertices are masked.
Tip: To quickly lock weights for all but a few influences, select the influences you want, click
Invert Selection

to select all other influences, then click the lock button

on any of the selected influences. The weights for all selected influences are locked, letting you paint weights for the original few influences. See also
Lock smooth skin weights.