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

1. Select a smooth bound mesh and open the Paint Skin Weights Tool.
2. In the Paint Skin Weights Tool, set the Mode to Paint Select.
3. Paint to select only the vertices you want to keep open for weights, then switch the Mode to Paint.
4. Paint weights.

   As you paint, only those vertices you selected can receive weight values, and all other vertices are masked.

TipYou can also use the Hold buttons to quickly set specific influences you want to work with. For example, to hold the weights for all but three influences quickly, select the three influences, click Invert Selection to select all other influences, then click the Hold button . The weights for all selected influences are held, letting you paint weights for only three influences.