If you take any vehicle on the salt no matter what precautions you take, it it is doomed. You can coat the underside with whatever you want, but salt dust will get everywhere not just the bottom. The salt particles are suspended in the air all the time, salt water splashes everything, you will bring it into the cab with your shoes and clothes.
The only solution is to make vehicles out of 100% plastic or ceramic and not have a bit of metal in it. Maybe one day...
Don