Why must rinse the outside of Visking tubing thoroughly with water?

To remove any residual glycerol.

Glycerol is often used as a plasticizer in the manufacture of Visking tubing to make it more flexible and easier to handle. However, glycerol can interfere with the absorption of water by the tubing and affect the results of experiments involving osmosis or diffusion. By rinsing the outside of the tubing thoroughly with water, any residual glycerol can be removed and the tubing can be effectively used for osmosis or diffusion experiments.