Dust and road grime collects inside body panels just like it does on the outside. When you're washing the car, some of the water gets inside the panels and drips out over the next half hour or so. That wash water picks up the dust and grime from inside the panel and slowly leaks out any weep points like door handles and bottom panel edges. Realistically, there are four ways to stop this: 1. Wash the car so often that grime never gets a chance to build up inside the car(this may or may not be possible due to a person's driving conditions - if you drive on a gravel/dirt road, you'll never be able to keep it that clean). 2. Wash the car thoroughly enough that the water that gets into the panels eventually flows clear(this can be a bit dangerous as, though it's safe to have a little water in those panels, basically flooding that area may not be good for any electronics). 3. After the car is washed, use a blower to force the moisture out of those areas, then immediately wipe off any dirt while it's still wet(wiping off dirt when it's dry can scratch your paint). 4. After the car is washed, come back in 15-30 minutes with a waterless car wash spray and wipe away any dirt. Same thing if you recently had a downpour that caused some dirt to wash out. Strangely enough, I just discovered there's a product out there specifically for this problem. It's meant for detailers who need to keep the panels clean while they're applying coatings and the like. It's a microfiber strip with magnets sewed up in pockets. You place one in troublesome areas(under side mirrors, fuel doors, door handles, etc.) and it catches any water that leaks out after a wash. MAGNETO - WORK STUFF | Car Detailing Products
