First is to always wash from top to bottom using 2 bucket system if do from home.
Second, the idea is not to scrub the door panel.. hose it down with a plenty of water, not neccessary use high pressure jet. The water will remove any sand or pebbles.. fyi, sand does not stick on the panel that water cannot remove. Tar maybe...
Third, use a good mitt and car shampoo.. apply generously on the door panel.. not scrub, just like normal washing. Unlike some opinion, I think a good shampoo helps.. it works much better than cheap chap XX. FYI, Meguairs NXT Gen or Mothers Gold Class should be considered.
Fourth, hose if off with again, plenty of water.
Fifth, dry if off with a good microfibre towel. I use the hold 2 ends and pull over the panel method.
Hope this helps.