What Pedro said. There is still LOTS of salt in/on the dry sand, and lots of salt in the air.
All this reminds me I need to give the vehicles another coat. I used to use fish oil with the following procedure:
Buy 4L tin of "deodorised" fish oil
Park car on plastic painter's dropsheet
remove door trims (at least at the bottom)
Turn on air compressor
Fill degreasing gun (long nozzle type) with fish oil
Spray fish oil into every nook and cranny I could find. Usually took 2L min to do a whole car.
It takes ~ 2-3 weeks for it to dry completely so sand won't stick.
Over here (WA) I have been using lanotec, it seems to work well also.

