A BE-400XP crew could not see the runway at minimums; resulting in a missed approach and diversion. Reporter stated the rain repellent system failed to clear the windshield during heavy rain.

Narrative

Upon arrival at destination; a diversion was executed to low ceilings and heavy rain. The visibility and ceilings were reported such that an approach should be successful. However; after reaching the MDA and MAP; we could not locate the runway. It was mostly due to the lower than reported ceilings; yet the visibility through the windshield was so poor due to the rain; it would have been tough to see the runway with high ceilings in a light-heavy rain condition. The repellent had obviously failed to shed any water to make visibility acceptable in amount of rain. Not sure if there is a schedule for reapplication of the special rain repellent used on the BeechJet planes; but it would be a good idea to make it part of a routine maintenance cycle (say; between a certain duration of time on the plane or calendar time). Also; if there was a recommended test we could perform as a flight crew as a preflight inspection when accepting the aircraft or before flight to/from airports with forecast or actual rain conditions; that would help alleviate a possible emergency situation and perhaps any unnecessary diversions.