I spray the underneath of my rig with Fluidfilm every Spring before I hit the beach, it does seem to help. After I get back from every trip I take my powerwasher and use an undercarriage spray bar I got from Amazon and modified it to spray better. I find the 4000 PSI of the powerwasher helps to get the sand out of those hard to hit places.
I wish we could use UTV's up this way on our beach but they are not allowed, that would help ALOT getting to the narrow spots on our beach here in MD. This past year we have lost quite a bit of beach due to storms, which hopefully will be built up again over the winter somewhat.