All uses permitted in M-P districts shall be conducted within completely enclosed buildings, except (A) the parking and loading or unloading of vehicles, and (B) electric substation. All open areas used for parking or vehicle loading or unloading of vehicles having a manufacturer’ s gross weight rating, as defined by the State Vehicle Code, greater than sixteen thousand (16,000) pounds shall be enclosed by a wall or fence not less than six feet in height, with gates at all points of ingress and egress. All open spaces shall be graded and adequately drained, and shall be continuously maintained in a dust free condition by landscaping or planted ground cover or by paving. Except as a temporary use, regulated by Section 17.52.480, use of a mobilehome is not permitted. (Prior gen. code § 8-50.9)