On non porous materials such as PVC you can lose a lot of chemical by spraying at height as it just runs straight off and hasn't as much opportunity to dwell on the surface and dissolve grime . A light flocked brush holds the chemical and allows better coverage especially if detergent is present in your mix as a surfactant. A pre spray doesn't hurt before working chemicals in via brush and backpack , dwell then rinse, then go over again where needed. But spray & rinse with no agitation from a brush is rarely enough to finish job to acceptable standard.
