Howdy
I work for a Gov Dept in QLD and I manage a number of LC'S that are on the beach for the life of their lease, we use one of the electronic products plus a septone product of which the name escapes me, it is a tar based product. So far we are really happy with the results on the chassis etc and is resistant to sand blasting the under carriage gets.
However I think if we did nothing other than wash the cars down every day, I don't think that the chassis under body would be significantly affected (other than surface rust), for us it is the things that you cant protect that die, things such as alternators, starters, electrical plugs etc. Interesting our oldest vehicle (16 months) has the most rust damage to the bulbar and the cnr of the door panel where it meets the cnr of the quarter glass.
I can reccomend the septone product which is similar to a number of other brands (tar based), but my biggest tip is to NOT drive in the wet sand areas on the beach and NOT to drive through the ocean. (much like you see in a certain 4WD magazine on a regular bassis)
And above all, wash the car as soon as possible, use low pressure high volume not high pressure low volume ie gurnie.
Good luck,
Ben

