get a stainless steel bucketI don't think that is a battle you will ever truly win, especially with salty sand.....
The only thing you can really do is ensure there are no pockets for the sand to sit and create rust. Washing it with fresh water after a days work would help.
Spraying thinned out oil woild help keep the rust down too. Dilute it with kerosene or possibly just use diesel.
i actually think the best way to keep a bucket from rusting is to use it. it's always getting worn cleanWhen you're done using the bucket put it under a roof or shed if you have one. Other than that it's a piece of equipment, rust happens if you can't keep it inside.
you can spray it down with diesel fuel , red / farm fuel is cheaper get a 10.00 pump sprayeri actually think the best way to keep a bucket from rusting is to use it. it's always getting worn clean