What should be applied to one windshield at a time during rain repellent application?

The most appropriate choice for application during rain repellent procedures is fluid. This is because the rain repellent needs to be evenly distributed over the windshield surface to ensure proper adhesion and efficacy in repelling water. Applying fluid directly onto the windshield allows for an adequate coverage that maximizes visibility and minimizes the risk of streaking.

Applying water does not enhance the repellent properties; instead, it could dilute or wash away the rain repellent, rendering it less effective. Heat may be counterproductive as it can cause the fluid to evaporate quickly before it has adequately bonded to the glass surface. An anti-icing solution is specifically designed to prevent ice accumulation rather than for enhancing rainwater removal, making it unsuitable for this purpose. Hence, fluid is essential for achieving optimal results when applying rain repellent to the windshield.