To fix stiffness in 3D model weight painting, smooth weight transitions between bones using soft falloff brushes or blur tools—this reduces sharp influence boundaries that cause rigid movement.
Stiffness usually stems from abrupt weight changes: vertices fully controlled by one bone (no blend). Gradually blending weights lets vertices respond to multiple bones slightly, enabling natural motion (e.g., elbows bending without creasing).
Check for vertices with 0/1 weights (locked to a single bone) and adjust to add subtle shared influence. Test the model in poses as you paint to spot stiff areas early—this avoids reworking later.